I've just bought my most expensive rig to date and I have never used a UPS because I've never really understood what it does other then supply power in the event of a black out, which I really don't care about too much because I can pick up the phone and call my broker and close anything I need to in 30 seconds. Is there any other "real" reason I should get one ?
Yes, a good one keeps your power from suffering hiccups like brown-outs and under/over-voltage situations which can really do a number on your PC. Having your hard drive lose power while its updating critical data can really mess it up and take a long time to recover from (short of reformat/re-install). Its cheap insurance. I have them on all of my PC's (and networking gear, and home theatre gear). Even the low-end APC ones that go on sale for $25-$50 are better then nothing.
depends on quality of electricity in your area, but-it never hurts to have one, just like GTS said http://www.pcmech.com/article/tips-for-buying-a-ups/
Has their working life improved? Seems I read before the battery lasts only a couple of years then the unit needs to be replaced. That right?
Yes, that seems to be a funciton of lead-acid battery technology (they only last 2-4 years). Not surprisingly if you keep them in a place that gets warm the battery life is greatly reduced. I buy replacement UPS batteries off of ebay - they are much cheaper that way (even with the shipping)
Lightning was causing me some problems which the ups seemed to help. My ups has a replaceable battery, but it is nearly the cost of a new ups.