In the C caching pointers case, the issue is that the system makes no guarantees about the "live"-ness of any prior pointers, whereas on the web this is entirely possible and encouraged (See oft-cited post about "Cool URIs don't change)
How can that ever be a realistic expectation over anything but the immediate and short term? Not to mention that in the overwhelming majority of cases the "provider" of a URL is not the party responsible for maintaining it.
I have to agree with the parent - unless the recipient of the URL [continuously] validates the source, she is simply asking for trouble down the road.
Trusting a website doesn't mean you have to trust them indefinitely. You can trust that the URL will be kept alive for a certain length of time - minutes, hours, days, etc - and deal with them accordingly.