This got discussed at length on the board now called "tea" a long time ago see [Questions spoiled by comments](http://tea.mathoverflow.net/discussion/950/questions-spoiled-by-comments/). 

In brief, the key points were (as I see it):

1. Some people sometimes prefer to answer in comments, and there is not much that can or even should be done against it (except perhaps to point out some negative side-effects it can have on occassion, which you just did).

1. It is for a third user or also OP of question alright or even encouraged to turn comments (with attribution) into an answer; if it is nothing much more than copying one should likely make it CW (as discussed there). 


Or, if one can answer the question by oneself there is, at least in my opinion, also nothing wrong with just answering it (even if a comment answers it too). It is somehow not done so much on MO, but the general SE model is anyway that there should be, or at least there is nothing wrong with, there being multiple (even if in part redundant) answers.