Should we let the actions of the stupid preclude normal people from having fun? I've smoked marijuana on and off for a few years, and I have no reason to believe that my cognitive functions have been impaired in any fashion. Just because there are those that smoke marijuana who are stupid, does not mean that everyone who does is stupid.
Traditionally, alcohol has been a more accepted drug due to it's legal status and deep roots in most European cultures. Weed does not have the same kind of status, and many know little about it's effects. Due to a series of political reasons, marijuana has been illegalized and combated by the War on Drugs. Thus, the "normal" people, being law-abiding citizens and being told by the government that it is bad demonize it and refuse to have anything to do with it.
The people who -do- do marijuana end up being the kind of people who live their life looking for the next high. These kinds of people generally do not value education, thus propagating the "stupid stoner" stereotype. In my opinion, if we were to legalize the drug, it would slowly gain social acceptance and, in turn, intelligent users.
And though there would still be those who abuse it, there's not really much that we can do about that. Better drug education would be a start. There are plenty of people who are cripplingly addicted to alcohol or cigarettes, and marijuana can have the same psychological effects over time. I know several people in my family who are vicious and moody when out of weed. But that doesn't mean it happens to everyone. Used in moderation, a bit of marijuana is fine, and the government should respect that.