This could be quite nice for newcomers and also when wanting to respond to a question with typesetting one could see how they did it and imitate.
Of course there's the markdown guidelines, but seeing something you like and copying it is much easier than reading through a manual. Also, this is in line with how I learn LaTeX, see something nice, read the code, and incorporate.
For example, there is a question on math.SE about a strange equal sign that I have no idea how the person wrote. It would be nice to be able to see how it was done (somewhat like in MediaWiki where you can "view source" even if you cannot edit).
Addition: I've added a [feature-request] in meta.stackoverflow for a "view source" button. If you have an opinion about this, please vote or comment or answer over there.