there is a big difference between marijuana and other drugs. also, you need to understand that everyone is different and is effected in different ways.
some people dont drink coffee often, so if they drink one before a gig they could get jittery and anxious and effect their playability. does that mean everyone who drinks coffee before a gig is going to be sloppy.
and the fact that it makes you "stupider" is an old wives tale. there are thousands of well educated people, who contribute greatly to their communities, life normal everyday lives and smoke enough pot to knock most people. it is completely unfair to associate marijuana with childish behaviour, sloppy music, and unintelligence. I can think of a few music professors that i have met in my day that were some of the most interesting people i have encountered in my life (unreal player's as well) who were very open/avid marijuana users.
so in conclusion, it may not work for you, but try to have a little bit more of an open mind. just because i like my grass does not mean that i dont work hard.