Here's a situation:
A reputation hungry newbieperson sees a comment (from an old useranother person) that offers a (sort of) correct answer for the question. He doesn't understand it well, yet he posts it as answer in the hopes of gaining a lot of +10's.
Now, that answer is partially right (it's been copied!). What would you do to that answer? Removing it means leaving the question answerless. Keeping it would mean a low quality answer and unfair to the one who posted the right comment. I've seen such situations, even though I haven't been on SE for long.