Well, as long as marijuana is illegal, people who break the law and smoke it will smoke it wherever the hell they want because they're breaking the law anyway. Whereas alcohol is legal but it's against the law to drink and drive. Why can't there simply be laws to prohibit use of pot in certain situations?
I think your personal experience has a lot to do with your stance here, which is understandable, but I would hate it if laws were based on the personal lives of the lawmakers (which a lot of them probably are, but I digress). I'm sure you don't believe that fatty foods should become outlawed, and yet it's claimed more lives than marijuana ever will. This example is sort of overused in this thread, but probably because no one on the other side of the fence has managed to dispute it. Something being potentially psychologically addicting is no reason to ban it because ANYTHING can be psychologically addicting; how many people waste their lives watching TV and playing video games? I tend to agree with arXter that it's a case of people who have abused a substance ****ing it up for everybody else who just want to have some fun and won't develop any serious problems.
If I woke up tomorrow and sex was outlawed because of STDs, and there was a thread created about it here where someone who has crabs came in and was all "no guys I agree with the new law because you have no idea how painful it is when you get a sexually transmitted disease," I would probably want to punch said dude in the nuts.