Pot can be a very powerful and useful thing if used properly. I have much experience, once chronic, now very sporadic. When used everyday it has the negative effects on memory and to some extent comprehension, however, when used sporadically and with proper dosing (Giant bong rips are not proper doses, BTW!) it can be a very powerful tool to open your mind up to different ways of thinking. Historically cannabinoids have been used for this purpose for millennia. It is especially useful in opening up creativity, or maybe just unencumbering creativity that is stifled by conscious thought. In small doses there is nothing better for musical experimentation, too much and experimentation becomes futile as your memory will be hampered. I've lost track of many transcendental pieces of music that way. This stifling of the conscious thought bodes well for pool players where the conscious self can get you into trouble, too much though and you wont be able to formulate proper strategies.
Pot is not the same as any other drug aforementioned in this post. When abused it can be a bad thing, not as bad as alcohol, IMO. When used properly it can have great benefit. And let's not forget the medical benefits.
Many players back then had some sort of "mixture". Pot was then, and continues to be, part of that mixture. Greenleaf was a druggie, as well as an alcoholic. He was also debatably the best poolplayer that ever lived!