There are studies that show links between marijuana use and schitzophrenia/psychosis, especially when MJ is taken in adolescence [1]. The evidence is not conclusive, but to say, categorically, that marijuana does not damage the brain, is quite a reach, especially when you are claiming benefits (reduced cancer risk) based, at best, on similarly non-conclusive studies.
All in all, your post suggests, well, that you might have been smoking something. At some point, anyway. Hopefully not as an adolescent. Whatever, I'd suggest that it's main benefit is as a relaxant, but that's based on heresay.
[1] No, I don't have links, but the other half just finished a year studying brain function and the effects of drug addiction and ageing, for her Masters, and I've been hearing about it constantly. Trust me - it's out there.