I actually like this idea, if only because there are other SE sites where the ability to hide a portion of one's answer (until the user elects to see it) would be beneficial. (I'm specifically thinking of SmarterGamer, where the ability to hide spoilers would make a lot of sense.)
On StackOverflow, I'm not so certain that people want to hide portions of answers, nor that users want portions of answers hidden. Thus, I'm less certain of its utility. (The one use case that really does spring to mind is when you have to post tons of code for some reason -- the ability to hide everything except the error lines might make sense; those who need the rest to debug can still get to it ... Er, I still don't know.)
That said, I can tell you that one of the owners of the Trilogy happens to be very much against code folding in general, the
#endregion pair, and posting large amounts of code to SO. So I wouldn't exactly hold my breath on this one. Indeed, you'll notice on link #2 there that this very same request is ...
[status-declined]. (Yes, your rationale is different, but the underlying tech is identical.)