Not that the NIDA has much credibility in the first place, but did you read the page titled "Is marijuana a gateway drug?" at the NIDA source you were so kind to link to?
let me help:
The gateway hypothesis continues to be unsubstantiated, and even the NIDA with a history of bieng very loose with their "facts" for the sake of propaganda say as much.
here is a link to a post where I take on one particular example of NIDA's dishonesty for the sake of propaganda:
http://www.debatepolitics.com/polls/116356-marijuana-30.html#post1060102977
Even if marijuana is dangerous, that does little more than strengthen my feelings that it should be legal. I do not want drugs legal despite their dangers, I want them legal because of their potential dangers. We can control distribution, make it less available to youth, and delay the average age of first use (which is a HUGE factor for whether or not someone will develop substance abuse/addiction in their lifetime).
We are not doing anything about our drug problem in this so called war on drugs, we are dealing with a manufactured criminal problem. Without the criminality we can take steps to deal with our drug problem head on instead of wasting time, money, and resources for a criminal problem that exists as a direct result of prohibition.