"Plus, it's only possible to choose one answer anyway."
Perhaps I'm being overprotective but when I try to put myself in the learners place (and forget all my own knowledge of HTML forms) I thought that:
even just momentary uncertainty is something you'd like to avoid in a testing situation
someone could click one radio button, then another, and possibly not notice that the other had deselected itself (this is possibly outlandish, though in longer lists of answers still possible, I think)
Note also that I'm talking about replacing the current "Answer:" text, so it would be obvious that the text was part of the Moodle system, and if it did seem redundant for some questions that logically can't have two answers then it wouldn't look like the person creating the question text was responsible for the redundancy.
But the more likely causes of trouble is the possibility of the testee not realising that multiple answers are possible unless told so, which I've just done for each multiple answer question in my Quiz. I assume everyone else does this chore too.
I also thought about your suggestion of further limiting answers e.g. maximum of 4 and a minimum of 2 answers and having a) that enforced, and b) automatically displaying appropriate text
It would be quite a job though as HTML forms don't support such niceties natively so you'd need to cook something up with Javascript as well as checking it on the server.