Many argue the legalization of marijuana, so I ask you why you would not want it to be legalized? It helps slow the growth of cancer cells and it is helping places who have already legalized it gain money from being able to sell/tax it.
What negative effects does it have? It may put your brain cells to sleep but it doesn't even kill them. Heroin has a LOT of negative effects which is why we shouldn't condone it. Someone who does heroin while pregnant then makes the baby addicted to it where if you smoke weed while pregnant, there are no known negatives to it.