This is not rare at all, I use it almost every time I link to a wikipedia article, for example: https://en.wikipedia.org/wiki/Newton%27s_method#Examples

That said, it seems easy to make this backwards compatible: if the existing #blah syntax is a valid link, that should take precedence.

FWIW, I think there's way too much cynicism in this thread. My first reaction on seeing the title was "cool! I would use that!".

edit: Hmm, I may have misunderstood the compatibility issue.