What about all the deaths the US Military causes through war, and fighting two wars at once is very costly and stressful to the US budget?
I'm going to save myself a lot of time and effort and copy for you the response I wrote to a very similar question that I feel covers what I would say fairly well:
1. the Middle East remains a strategic center of gravity in the world for two major reasons: the oil and the canal, and huge chunks of the world economy are dependent on both of those. instability in the region threatens those two facets, thus threatening the world (and our) economy.
2. the Middle East is inherently unstable, as demonstrated by nothing better than recent events. Tyrannical governments keep their populace in line with the stick of the mukhaberat and the carrot of the welfare state based on revenues generated from nationalized resources (read: oil and the Suez). But that rentier state carrot is intensely vulnerable to falling revenues and - as the Iranian Shah and Mubarak learned to their chagrin - can rapidly inspire revolution followed by replacement by radical (and themselves inherently destabilizing) elements. Internally, the Middle East is a bubbling cauldron, and the resources upon which much of the worlds' economy is based right there in the middle.
Internationally, among the Sunnis, Egypt and Saudi Arabia both consider themselves the natural leaders, and have already proven willing in Yemen to shoot at each other over that disagreement. The Iraqi's also consider themselves the natural leader of the Arab world, but lately they haven't been a serious contender. The Saudis are currently attempting to take control over the region through the exportation of Wahabism, which is itself inherently destabilizing, as it preaches the overthrow of the National-Socialist model governments left over from the 60's and 70's in Egypt (check) and Pakistan, (as well, obviously, as the democracy - as much as it exists - in Lebanon and in Israel) followed by the violent unification of the region under a single banner, followed by an invasion of the rest of the world. They aren't kidding about that part, and we are idiots if we fail to take them at their word, especially as they seem to have just succeeded in part A of step 1, the removal of the Mubarak regime.
The Iranians are the largest terror-exporting nation in the world, and they are very, very good at it. The IRGC, and in particular the Quds forces, have fostered the growth of Hezbollah (the real deadliest terrorist network in the world - Al Quada was their student, not the other way around), Hamas, and even (through proxies) Al Quada. They are currently waging a campaign to destroy the Lebanese government, and are strengthening ties with Syria and Turkey in an attempt to build a base with which to challenge the US and Saudi Arabia for dominance of the region, part of that struggle (they assume) including the destruction of Israel. The leadership of that nation Really Believes that the 13th Imam is coming soon, and that they must kick off international Jihad in order for him to arrive and bring about the End Times - and again, we are fools if we fail to take them at their word on that.
3. the region, thus, needs an overpowering, hegemon if it is to remain stable enough to ensure the non-collapse of the world economy. Someone has to impose order and keep these nutjobs from destroying the ability of the world to access the oil and the suez. There is only one nation currently on the planet with the capacity to perform this task: the US. The US Fifth Fleet, currently headuquartered in Bahrain, is the major (and perhaps only realistic) force for stability in that region, contending with numerous, powerful forces for instability.
4. Withdrawal or severe downdrawl of US Forces would create a power vacuum and kick off fights within the sunni community and between Iran and Saudi Arabia for regional dominance. Shiite Iran is seeking to get nukes. Syria has had a nuclear facility already destroyed by the Israelis. Sunni Pakistan (see: Wahhabi plans for governments, the overthrow and replacement of) already has them. In the face of a US Withdrawal, Saudi Arabia certainly would start developing her own.
Imagine a Mexican standoff, except that 3 of the 4 players are A) paranoid schizophrenics facing opponents they violently hate, B) convinced that death will be a net benefit for them, C) convinced that their souls are in peril if they don't shoot, and D) potentially armed with nukes (the 4th Player is the unfortunately-located Israel). I think everyone here can agree that that is not a "stable" situation, particularly when you add in E) these countries are not internally stable, but may feel forced into an external war in order to solidify internal support and F) at least two of the players (Iran and Saudi Arabia) are held hostage by their own extremists, who feel free to act without permission, are nearly impossible to stop, and are most desirous of the conflict. And I feel that A) deserves rementioning.
FUN FACTS WORTH NOTING: China (also nuclear) is rapidly becoming a good, good friend of Iran, and is semi-distancing itself from Pakistan (whom it largely views as a foil against India). China is also heavily invested in East Africa. It is possible that China would seek to intervene in the region to tilt the balance in Iran's favor as the US did in Saudi Arabia's. If that happens, then the newly Taliban (and nuclear!) Pakistan - which is deeply paranoid, xenophobic, and a wierd mixture of Wahhabist and neo-Deobandi - becomes an ally of Saudi Arabia, and our players are all now holding two pistols even as their inner demons scream at them to shoot first. BEST CASE SCENARIO here is that China is able to stabilize (kinda) the region, and merely takes all the oil for itself - only partially collapsing the world economy. but that's the "best" case, not the "most likely" one. it's not even really a "sorta likely" or a "semi likely" one.
5. The West is dying. Literally - our creation of an entitlement culture and our devotion to materialism have left us with birthrates below replacement level. In both Europe and America the solution has been mass immigration - but both have had issues with assimilation. America here is comparatively lucky, her immigrants share many of her cultural assumptions. But Europe is not - the West in Europe is being replaced by a high-birthrate Islamic culture which does not accept the Enlightenment. As the immigrant populations threaten to break the local safety nets and culture, the backlash they provoke isn't what we would recognize as classic liberalism, but rather classic fascism. Nationalist groups are springing up all over Europe, though they are doomed by their own inability to breed to dying out after sparking conflict. All those aspects of the West that we consider dear; the rights of the individaul, limited, secular government, free markets... they are doomed to wither and die as the culture that upholds them does.
The situation at current cannot sustain indefinitely - eventually the destabilizing elements that are currently inherent in the Middle East will win, and the price of loss is not just a world wide economic collapse, but the slide, decline, and perhaps fall of the West. The long-term solution is therefore to change the rules of the game. The destabilizing elements in the Middle East must be replaced with stabilizing ones. Tyrannies must (carefully) be replaced with representative governments that give public pressure an outlet other than violent overthrow. Rentier societies that encourage stagnation, revolution, and hostility abroad must be replaced with market economies that encourage trade, growth, and a politically active middle class with a vested interest in stability. Radical Islam must be replaced with a new ideology that allows Muslims to recoup their pride and independence without striking at others. In short, we need to allow the Enlightenment to do to Islam what it has done to Christianity.
Even with our presence, US pursual of that strategy (again, as we see today) is not guaranteed, and even with US pursual of that strategy, sucess is not any kind of certain.... but if the US withdraws before these things are accomplished (or, at least, accomplished enough to become self-feeding cycles), then the game is up. the match is struck. Europe falls, China moves to become hegemon, nukes possibly fly, and back to the Dark Ages we go, but this time with much, much better weapons with which to massacre each other in the name of God.
THAT's why i would suggest that "oh well let's just leave and let em fight it out amongst themselves" is a bad idea.
I also find it interesting you didn't seem interested in discussing the particulars of the "how".
