I think a lot of people come to the mistaken conclusion that smoking pot makes you "stupid." I don't think it necessarily impairs cognitive performance as much as it makes it seem less important. If we were passing around a joint and before I had to hit it again, I had to multiply two 3-digit numbers together and translate a a paragraph into spanish, I could. But smoking makes you less likely to approach a problem like that. It allows you to take in perspectives other than your own.
While smoking, I have uncovered repressed memories of childhood, resolved interpersonal conflicts, and redesigned interfaces.
Calling a high person "stupid" is like calling someone stupid because they speak a different language, or calling a child stupid because they ask a question about something that seems trivial.
I've always felt that weed helps one reconcile disparate concepts. When you take off all your different hats and just look at the problem with a more basic human perspective, without the little compartments into which we organize all our issues. Of course, some people can achieve this level of perspective without drugs, more power to them. Similarly, some people react differently to marijuana - bummer for them. I smoke regularly, but not when I need to code - it's not that I can't code, it's just horrendously unappealing when high.
The bottom line is just knowing what helps you accomplish what and when to take part. I speak significantly better french with a little booze in me and I design better logos after a few hits. Some people are better designers and french speakers than me without ever having tried either.