There will always be a criminal eliment attracted to an illegal activity in order to make money. When alchohol was illegal, bootleggers came out of the woods. Pot is illegal now, so that's where it goes. If pot were legalized, they'd move on to the next "gateway drug".
I believe some level of graff is necessary in society to keep some elements of society occupied. Unfortunately, there's a difficult balance that needs to be maintained. Tampering with the balance in the early 1900's got us prohibition and caused unpredected events that greatly outweighed the consequences of leaving it legal. I think that tampering with that balance in the 2000's (in regards to legalizing pot) could have similar effects of an unforseen nature.
People tend to oversimplify this issue and the implications when these type of discussions occur.