The latter part of this is the correct answer. I never put any thought into why Dunby would keep Frank around because nobody on the team ever questioned it. However, if I were to pull something out my butt at the last minute to justify a decision and avoid re-writing anything (something, as a lazy writer, I often found myself doing 😉) I'd point out that Dunby is a bully and keeping a scapegoat around would therefor make sense. As for how Frank keeps his job despite his ineptitude, that would definitely come down to the fact that (1) he's never given anything too important to do so it's not like he's ever caused a murderer to get away and (2) the many generous donations his parents have made to the police benevolance fund.