I think it very much depends where you live. When i've been over in the States (e.g. this summer in CT) the lights were flickering and dimming every few seconds, I was like
In the UK we don't get anything like that and only have a powercut maybe once a year, if that, so I don't have one.
There are voltage surges and general voltage irregularity. Having a UPS gives you a MUCH higher level of surge protection and it helps when the power fails. But power failures alone aren't the only reason to have one. I don't ever have lights dimming or anything like that but I still run a UPS.
