This is related to this question: On the inclusion of pages-of-text-as-images in questions and inspired by this answer of mine: https://math.stackexchange.com/a/100984/2614
So, the discussion in the other meta question specifically addresses the inclusion of images in questions. What about answers?
In the answer I linked to, my motivation for posting a full proof out of a textbook is the following. I believe the question posed by the OP is very natural and deserves a clear exposition; however, I remember the proof of the fact being quite involved. I remembered reading it in Warner, so instead of just linking to Google Books, I thought it would be nice to have a complete, readable answer to a good question in math.SE. But it somehow makes me uncomfortable, answering a question like that (not that I would do it frequently).