I suppose if this feature were implemented tastefully (uh, ok, I guess the concept of public urination is sort of tasteless in and of itself, but oh well) it might make for an occasionally funny addition...I don't see any way of doing it that wouldn't call for this game's "Teen" rating to be changed to a Mature though.
Interestingly enough, piss is actually pretty sterile stuff, consisting of about 96% water with the balance made up of insoluble vitamins and the like. Ounce for ounce it's something like 25 times more sterile than spit. Some people believe that (warning: the end of this sentence is pretty sick...and considering how sick it's been thus far, that's pretty bad) drinking one's own urine somehow detoxifies the body. An estimated 1 million people in the US alone (I think? or was it worldwide?) consume their own piss daily...ewww...though to give them the benefit of the doubt, I'd assume it's served at a refreshing 98.6°.
But let those guys do it...as funny as it might be in BF2, there's no reason or place for it there.