No, I'm not thinking of cases of gracefully closed sessions, only
the behavior of non-gracefully closed sessions.
I think the lack of argument here means it's by and large not an
important issue, since folks shut down sessions gracefully before doing
a maintenance, right?
But the answer that "it needs to be at 180 seconds holdtime to maintain
routing stability" doesn't seem to hold water since no one's claiming
instability caused on private peering links and iBGP sessions due to
10 second interface keepalives 
I think the answer is that it just hasn't been important enough to
ever change the defaults.