Well, the watchdog should be simpler than the uC. That means that its failure modes are easier to examine, and that it's easier to design it to be fail-safe, i.e. that failures will lead to the watchdog triggering a reset instead of the watchdog locking up.

Of course, a watchdog that can have its clock turned off by the uC performing a simple write isn't going to cut it it situations where you can't expect the user to remove and reinsert the battery. It might be ok for game consoles, music players and cellphones, though.

Jack Ganssle wrote an interesting article about watchdogs and how to identify good ones and bad ones:

http://www.ganssle.com/watchdogs.htm