Almost everything is (potentially) linked against WebKit (At least judging by how broken my system when a rogue rogue flash plugin broke webkit.). Certainly Finder, for example. It could just get away with enforcing log off/log on again, but if multiple users are logged in it is probably simpler just to restart. "Ignoring at their own peril" will almost certainly result in horrible crashes.

Why would there be crashes? The running apps will keep their already loaded binary until they're restarted.

Maybe there'll be some bugs in handling shared on-disk resources (cache, and maybe something else?), but I doubt that those parts change very often. That would be a reasonable reason to require a reboot, but unless those parts actually change it should be avoided.

Sparkle (the update framework) uses WebKit, so everything that uses Sparkle links against it.

Ah, that brings back nice memories of IE and Windows.