I'll give you 4 Reasons why Cannabis should be Illegal
1. By keeping marijuana illegal you maintain the stigma which surrounds it. For a teenager to smoke it, he must be breaking the law, i.e., disrespecting authority, and that is attractive to teenagers. By legalizing marijuana you reduce the stigma, which then may transfer to a more dangerous drug.
2. If you make marijuana legal, more people will use it. Classic Law of Demand. Lower the price (consequences), and the quantity demanded will increase, including use by some who were priced out of the market before. If we claim people smoking marijuana is a bad thing, this means more of a bad thing.
3. People smoking marijuana makes other people uncomfortable. It smells weird. People who smoke it act funny. It just makes non-smokers uncomfortable. More smokers means more interaction with non-smokers, leading to more uncomfortable non-smokers.
4. It is hard to prove you are not high. Since the THC is stored in your fat cells, it may be difficult to prove you were not impaired by the substance (even after the 2-3 hour effects had worn off). For example, if you are in a car accident the day after smoking up and weed was found in your car, that may be grounds to test you. If it is found in your system, you could face additional criminal and civil penalties. This assumes that there isn't a BAC-impairment equivalent for marijuana testing (I haven't heard of any). The legislation and litigation that would result from these disputes would further tax the judicial system. This may be partially offset by fewer marijuana possession cases, but the net cost is uncertain.
Could I poke holes in each these arguments? Sure. Can I think of any others? Not at the moment. Feel free to add yours in the comments.