I know a shitload of people that smoke a lot of pot. They're late all the time, do 'just enough', etc.
I know some people that smoke a little pot, doesn't affect them much.
The people that need it 7x a day... I think it slows them down. Even if they succeed, they could probably succeed more without it.
You'll always have exceptions, but who's to say those who smoke a ton and achieve a lot wouldn't have done more, without it?