That's what I thought "only when it stops working" meant - doh!
My favourite answer to the question; 'When should I oil my chain?' is; 'Just before it starts squeaking'.
The OP's question is a very interesting one and the answer lies somewhere between riding your bike into the ground and only fixing things when they fail, and replacing all the components on the bike every two weeks.
With chains, sprockets and rings, cables, rims and blocks it's pretty easy to know when to replace them, since their wear rates are predictable and easy to monitor, but bearings? Without opening them it's hard to see how they are getting on. And you'd better know how to put them back or you can create more trouble than you fix. It's why grease ports are such a great idea. If you're an obsessive furtler like me your bikes wont get much of a chance to seize up, but one of the essential tasks is fixture maintenance - nuts and bolts, skewers and seatposts need to be checked for corrosion and greased up every time you move them, or at the very least, annually. Which is why stainless steel is such a good idea for nuts and bolts.
As for the rest of the bike, if it aint broke...