Weed is useful in so many applications, from paper to building material, cheap fuel, casual intoxicant etc. And has really no need to be illegal.
It's short term side effects are beneficiary alone, and its long term side effects only affect people with EXISTING mental disorders. The idea that it causes brain damage stems from an experiment done a long time ago when they pumped monkeys wearing masks with several hundred joints worth over the space of five minutes. It wasn't the weed that caused the brain damage, it was the lack of oxygen.
In fact, some studies have shown that in small amounts, weed actually BOOSTS brain activity, something I can relate to myself, as it has gotten me through some tough essays in the past.
It does not cause crime. Like any drugs, the only crime revolving around the drug is the fact that it's illegal, and therefore must be sold in a black market. With weed especially, it's one of the few that doesn't turn a person violent. Anyone who's seen anyone 'doped up' will understand what I mean.
Compared to alcohol and cigarettes, it is much more healthy. Not a single weed-only related death. Thousands related to the legal drugs, even the ones normally beneficial like paracetemol, aspirin, ibuprofen etc.
It is not a gateway drug. The high obtained from weed is nothing like the harder drugs available. The gateway is usually the dealer of weed, not the weed itself, who may offer someone who regularly picks up weed from them something harder. Haven't had this myself, but I have a friend who has been offered cocain on a semi-regular basis.
Now I don't agree that it should be legalised to the point of cigarettes, which can for the most part be smoked anywhere outside. Instead, it should have similar restrictions to alcohol, such as only being allowed to take it within personal property or designated smoking areas. Even then, it's hardly like a pot head will go around in a drunken stupor starting fights with anyone who even coughs in their direction. I was nearly a victim of a fight between to guys twice my age at the time (I was 16, they were clearly over 30) at 11am, and they were drunk out of their minds.
However, there are problems. Because the drug has been illegal for so long, the black market for it has grown huge. Legalising it puts a hell of a lot of people out of their 'jobs', from growers to dealers. So legalising it would be a rather delicate process to make sure these past dealers aren't forced into doing other, potentially more dangerous crime at the cost of taking away their weed business.
Video on Youtube, can't be bothered to search for a link, but it's called "The Business Behind Getting High". Very good video explaining most of what I have said here, and that's just the first half of it. Still have to watch the second half myself.