The substance is one thing, but volume is another. Are people smoking 'a pack' or two of marijuana every day for decades?
Personally, I think a big part of the problem is the tobacco in the joint. I'd like to see a safe or safer tobacco replacement for rolling joints with. I know they are other ways to consume pot, a joint is the most convenient. All the things you can do with a ciggy on the go, you can do with a joint on the go. <HNHumourbypasstag>Driving for example.....</HNHumourbypasstag>
There already is one: Marijuana.
The fact is you don't have to mix it with tobacco :) It's commonplace here in the UK but you can just smoke it on its own or you can use a vapourizer. You can even cook it with butter but that increases the effects