Here's an excellent read on the subject:
Ten Arguments Against Obama's Executive Action - AMAC, Inc. AMAC, Inc.
Excerpts:
Four: If you issue an EO making permanent residents or citizens out of a significant number (say, five to seven million) illegal, unentitled, or “undocumented” foreigners on U.S. soil, you are instantly obligating federal taxpayers and states to afford these newly minted “Americans” or “newly legal residents” any number of privileges, entitlements, and rights not previously held, above and beyond not being deported.
This plainly costs taxpayers and States money, offering them every reason to appeal the decision and apparent standing to do so. (IN EFFECT, IT'S A FORM OF TAXATION WITHOUT PROPER REPRESENTATION. ALL TAX / REVENUE BILLS ARE RO ORIGINATE IN THE HOUSE OF REPRESENTATIVES, PER THE CONSTITUTION AND NOT IN THE EXECUTIVE BRANCH. Origination Clause - Wikipedia, the free encyclopedia - Logicman).
Argument five: If you issue an EO that instantly grants “stay and work” status to currently illegal aliens, even if they have legal relatives, you instantly soak up part of the job pool from which real Americans are hoping to gain employment. In effect, you are hammering the working-class Americans again.
Six: If you issue this EO, you instantly send an international message – a new and shocking invitation: “We just gave away the citizenship or residency farm – so please line up or flood over and come get yours.” In effect, such an EO will trigger multiple future waves of illegal migration for economic purposes by new and unconnected illegal aliens who see that our laws are not being honored, and so they will come for free entry, too – if not at once, then soon enough.
Nine: Just as adopting a child into a home affects other family members, instantly making “legal residents” or “citizens” out of five or seven million people – many of whom snuck into the country unlawfully – would have profound effects on the rest of the country. It cheapens the brand we call American; it undermines the values and processes in which we take pride. It slights and diminishes the struggle of those who have strived long and hard to become naturalized citizens, or permanent residents, many of whom are also from these same countries. It says that laws under which we live are of less value, and can be unilaterally upended by one man. It reduces respect for all those who have come to our shores legally, and who take pride in being legally American. This is no small matter. We are, collectively, only what we say we are and live up to – when we cheapen the definition of American, we cheapen it for everyone.
Finally: We are a nation and people of laws, not of whimsy or capricious acts by self-adulating leaders, not subject to any dictator or the assumption of power by this or that president. These lines are well-drawn. The U.S. Supreme Court long ago made the point. We are not ruled by executive order, never have been, legally and prudentially cannot be, and should not now be. For any president to believe that he has the power to step upon all these legal and prudential considerations, because he has a pen and a phone, indifference to rule of law, or illusions of unilateral authority is simply misguided.