I strongly believe that Stack Exchange and its sites will last for a long time. It favors structure over convenience. Every contribution will be findable many years from now. The whole concept is very well elaborated. It is a pleasure to thoughtfully prepare a question and await its answer, thinking of others who might make use of it.
I suggest to forbid any external reference, especially URL's. Some of the worst cases: links to irregularly changed wiki articles; short-lived addresses generated by a CMS like id=23423; references to amateurish content; references to commercially distributed literature; references to sites demanding a registration fee.
If the author is too lazy or incapable of articulating the occasion, background or scenario, the question is not worthy of being raised at all. Stack Exchange is a superb tool and not some random bulletin board.
If and only if an external reference is given, it must be accessible to everyone at any time.
So, what do you think?