What you're thinking of is that by default, if the system has been running in degraded mode for 24 hours (i.e. if a drive failed 24 hours ago and hasn't been replaced either by a hot spare or a new disk), the system will shut down. I think the idea is to reduce the likelihoood of data loss caused by a second disk failure (and make sure the admin knows that something's wrong).
Our motivation was really your second guess:
To make sure the admin knows something is wrong.
We couldn't come up with any way to absolutely, reliably *guarantee* that the sysadmin would get notified except to have the box turn itself off. We even joked about putting lights and sirens on the box, but the problem is, at smaller sites, people sometimes stick these things in a closet somewhere and forget about them.
Of course, at many sites, especially larger sites, people have all sorts of notification methods (like syslog, or autosupport linked into an e-mail pager), so we do have an option to turn off the 24 hour shutdown feature.
Dave