pro: it makes sitting around the house doing nothing seem OK
con: that's called amotivational syndrome and can be a problem

pro: sometimes weed can inspire people creatively
con: can't remember what they were going to write/say/draw/paint/etc

pro: can stimulate appetite in sick people
con: munchies usually consist of cravings for the most unusual or least healthy foods

pro: illegal and therefore not chemically altered and tainted by tobacco companies
con: illegal and therefore possession leads to jail time

pro: euphoria while high
con: burn out after coming down (if you come down that is)

pro: ^slows Alzheimer's development in old people, making them less senile as they age
con: speeds brain deterioration in young people smoking too much

OK I'm to the point I have to start thinking about it. I don't like thinking; gives me a headache. Maybe I'll come up with some more in due time.