http://www.well.com/user/woa/fspot.htm
Go ahead and read this to see why marijuana isn't legal.
Some effects are accelerated heartbeat, memory loss, slows reaction time/concentration, lung damage. Oh and read the part about how marijuana smoke contains more cancer-causing agents then tobacco smoke.
I'm pretty sure cancer, lung damage, accelerated heartbeat (which leads to heart attacks), etc. outweighs some cell growth in the brain.