But, sometimes, things get out of "standard", "normal", "typical".
design for worst case, not typical. I have seen shit (sorry, know no better word) that was "designed" based on the 'typical' values in the data sheet.

However, the electrolythics in it might suffer from a slow death

Yes, but that is the point: if the supervisor bites, then you fix it, you do not say "horray, the supervisor will catch the next one as well"

The supervisor is valuable NOT to catch a problem with a "typical" design, that would be the result of a bad design, but to catch what could not be avoided, even with the best design e.g. component failure.

Neil Kurzman said it the best:
"The watchdog is the embedded fire extinguisher. You do not want to need it. But you want it there."
http://www.8052.com/forum/read.phtml?id=113613

one point: "worst case" does not, in all circumstances, mean a "welded down metal lid", common sense does apply. As an example we close our "lids" with conductive foam at the edges to limit RF emmiting and intrusion. The foam is may not be "needed" but to park a bus because of a sign failure due to passing a radio station is not good either (by law a bus is not allowed to operate if the sign is dead).

Erik