I only can find the time to do the core stuff, namely settle arguments and handling flags. a good mod should have the time to regularly scan posts, and upvote where possible, downvote and most of all, comment - especially for newbies.
I can do that over at Psych&Neurosci, where the traffic is low, and the relative number of active hi-rep users is high. At Bio traffic is dense and quality is often questionable - we need more hands on overseers. And that's, imho, why we need another mod, or two.