In response Starbuck and others above re: "at LACs we often end up teaching in areas outside our immediate research." I absolutely agree -- but your use of the word "immediate" is key to me. I am a comparativist at an LAC, and I feel confident that I could teach any CP course *competently*. Some CP areas are more foreign (haha!) to me, though. CPE is distinctly not my area; I could teach a relatively intro CPE course with a lot of preparation on my part, but I am convinced that students would be served poorly by my teaching a more advanced CPE course. This is where students at an LAC who reach more advanced levels in areas in which their faculty are not experts lose out. I don't think there's a way around this, given the size of depts at LAC. It's a matter of practicality, not a claim that any faculty member at an LAC can teach any class well. A poli sci department with two faculty members (though I don't think that's quite what you're referring to) is going to be a weak department, and the poli sci majors they graduate will reflect that.
So I'm down with people teaching any class within their subfield, with the caveat that this does not mean they will teach any and all upper-level classes *well*. And I'm down with people teaching the intro class in subfields not their own. My problem is with people teaching beyond the intro level completely outside their subfields; this is well outside immediate research, and I don't think it's good for students. For example, my own research, as well as my own graduate training, has literally nothing to do with IOs. Do I know what an IO is? Of course. I can point to basic literature, and I can even read up on what the fundamental questions (so far) surrounding IOs have been. But I would not teach as nuanced and as rich a course on IOs as someone who actually deals with them more closely (say, an IR person) -- not because I can't make the effort to read up on it, and not because I lack effective teaching skills, but because I don't have the expertise.
To the person who responded to Starbuck re: the colleague who teaches only redistricting in an intro AP class: that's a question of being irresponsible, not lacking sufficiently detailed knowledge to teach the material. I completely agree that professors, especially at LACs, must have a broad enough background to teach multiple intro classes -- but lack of this background is surely not the problem in the example you cite. The problem is that this person only wants to teach her/his specific research area, not that s/he doesn't have a grasp of basic intro concepts.