Jonas,
It's a wonderful thing to THINK the OS should handle an outage - and it SHOULD. BUT, reality differs. Reads and writes take place very quickly but if it loses power at the wrong moment, you lose the information.
Do you want your server rebooting because the lights flicker? Some servers can take 10-15 minutes to start up and then have to go through a disk check when they aren't protected. Users accessing the server via laptop and the internet won't necessarily experience a problem, but you could end up WASTING your employee's time or worse - if an employee is in the middle of a proposal and hasn't saved the file, you could lose business. UPSs are not very expensive in the grand scheme of IT costs and should be used in ANY place that is subject to even occasional outages. If you think the $100-300 isn't worth it for the server and $50 isn't worth it for the PC, that's your prerogative. But most people understand TIME is MONEY and any disruption in work flow will cost you money.