first off… everyone has a vice … sex, drugs, religion, exercise, reading, etc… most people do something i excess
2ndly - if used properly marijuana is extremely helpful
3rdly - if someone does abuse marijuana… that individual would of abused just about anything… at least with stoners they can still function in society without causing a ton of problems… sometimes marijuana helps those that have shit jobs to cope with them… and the world keeps on turning
4thly - using chemical structures outside of our biology aides our species in evolution and limiting it because people doing dumb shit… which is a factor in evolution… is not an excuse to ban it