I've been using a MP (1,1 and now 5,1) for nearly 6 years without a UPS, and I never ran into problem. I don't need another box cluttering my office! Unless you're running mission critical apps or huge simulations that can be paused gracefully if given a chance (saving hours, if not days of rework), a UPS is a waste of money, I think. Or am I missing something?
EDIT:
Of course, there are gamers among us, who just couldn't bear losing whatever it is that they got.
No one needs insurance until they do...just sayin...