While karma might help spam/bots in some ways, I feel like it would also lead to karma farming, which I’m personally happy to not have here. Maybe they could instead allow communities to set requirements for minimum time subscribed or minimum interaction (voting, commenting, etc.) before people could post? I’d prefer that be set per-community, though, and not a site-wide mandate.
That might be better, but I’d still worry about people rep farming (for lack of a better term). Any time you give people a score, title, or other personal metric, you run the risk of people posting to influence that metric rather than to post for the sake of contributing content.
It’s possible the good such a system could do would outweigh the bad, but it will definitely always have elements of both.