Honestly, I don't think they should test for it since it doesn't enhance his performance, plus some people are against drug testing because of privacy issues (like if they have leukemia), I think there's really no point in testing for it. Besides, it's not like he's responsible for anybody else's safety, just his own. MMA promotions like UFC and M1 affliction might want to do it just because they're paying these fighters a lot of money and want them to be healthy enough to fight, but that's more of the fighter's responsibility than the promotion. The only bad thing I see happening with Nick Diaz smoking weed is if he fights while he's stoned and still kicks the other guy's @ss (and after seeing some of Diaz's fights, that could happen) and it just embarasses any future opponent he may have.