Personally, I'd be more concerned about the possibility for psychosis....
Purely an anecdote, not at all measured and qualified, just correlation, but I've known two people over the years who've been long-term cannabis users and have had significant mental health problems. They're a pretty high percentage of those who I know have used cannabis or have had significant mental health problems.
I'm fairly sure there are such studies, but not longitudinal ones. I would say the reason is mostly to do with psychosis being a lot more rigidly defined than creativity, although there are obviously other biases.
How exactly would you go about measuring how creative someone was, then seeing if that had changed 10 years later, in a way that would lead to a statistical conclusion worthy of publication? A very difficult proposition.
A lot more work has been put into working out psychosis than creativity because it's a higher priority to treat the psychotic than to study creativity. And more grant money available. It's just a matter demand basically.
More grant money available, yes. But the reason for that is probably more political or cultural than intellectual or scientific. And a quick Googling shows there is plenty of research done by psychologists on creativity, too.
