The case where I wanted "routine" shutdown immediate (and I'm not sure I ever actually got it) was when we were using IBM HA/CMP, where I wanted a "terminate with a fair bit of prejudice".
If we know we want to "switch right away now", immediate seemed pretty much right. I was fine with interrupting user sessions, and there wasn't as much going on in the way of system background stuff back then. I wasn't keen on waiting on much of anything. The background writer ought to be keeping things from being too desperately out of date. If there's stuff worth waiting a few seconds for, I'm all ears. But if I have to wait arbitrarily long, colour me unhappy. If I have to distinguish, myself, between a checkpoint nearly done flushing and a backend that's stuck waiting forlornly for filesystem access, I'm inclined to "kill -9" and hope recovery doesn't take *too* long on the next node... If shutting a server down in an emergency situation requires a DBA to look in, as opposed to init.d doing its thing, I think that's pretty much the same problem too.
