You are not only upgrading Safari. You are upgrading Webkit as well! And it is used in many other apps, like iTunes. Webkit is very much integrated into OS X. That's why it needs to restart.
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.