I never had problems with power offs because I always save so frequently the documents that I'm working in, but if you think you are going to lose money in that way, no doubt about it, buy one.
Everyone should buy a UPS for their PC, I'm not using one before and my computer got busted. Having UPS is a priority.
Yes, i am using the UPS since 2 year with brand as a V-Guard system, Temporally it is not working. I think investors are best source of Back up.