I am against for various reasons.
1. Practical. I have/had certain *cough* problems with addictions, so I don't see why we should legalise another addictive substance while we already have one. Defenders of cannabis often, at least in here, use the "alcohol argument", showing that smoking abuse is not more harmful than drinking abuse. My question is: if we already have one substance which you say CAN BE harmful - what's the point of legalisation of another one?
2. Sociopolitical. I am of the opinion that countries, despite increasing egalitarism and "shift of power" towards masses, should promote certain behaviours. By "certain" I mean proved to be useful/worth/not harmful/generally leading to better results. Sure, everybody must have an opportunity to ruin his life for whatever reasons available (frequently called: freedom), but there must exist some forces who will use their power/capability to say: no, you are wrong. History/experience/our age/knowlege etc. leads us to the conclusion that a) is good and b) is bad.
Smoking cannabis certainly leads people to feel happier (according to some surveys in Poland 30% of secondary school pupils smoke cannabis on regular basis and reports claim that most of them smoke to reduce stress). Alcohol works a bit differently of course
3. Philosophical. I want to stop evolution, however I am almost sure that Scythe will pierce me with the sword for using the "evolution" term on these boards.

Important note: I never smoked cannabis.