Why would McCoy be so interesting to watch in command? Either he'd be incompetent, which would just highlight Kirk's strength or he'd be competent, and it wouldn't matter that it was McCoy in command because any other character would be able to deliver the same lines. So, what could possibly be interesting to watch? McCoy might swear more, I suppose...
I think it's obvious. "The Galileo Seven" is an episode about what happens when Spock is in command, how he uses logic and what he does when logic fails him. It was interesting. McCoy is the other side of the coin, an episode based on a similar premise may have been interesting as well.
If you think McCoy is just a generic character whose swearing is the only identifiable trait, I'd say that you probably haven't watched enough Star Trek.