How can computer parts suffer if the computer isn't protected by an UPS?
I'm not an expert on this, but I think there's a device here you haven't mentioned:  a power conditioner.  These devices take power from the wall and pass it through filters to make a (fairly clean) output power signal.  In some cases they also have automatic voltage regulation, and can boost/cut the power from the wall.  In certain cases, they are also tailored to the applications they are used in.  For example, APC makes a seperate lines of power conditioners and UPSes for home theater and computing, as they have different power usage profiles (home theaters tend to have high peak usage).

I don't have any hard numbers on this, but if the power signal is out of spec or the PSU you are providing with that power signal is poorly designed, damage may occur to the device.  Specifically what, I don't know, and others that know more will have to chime in.

Some UPSes perform a power conditioning function, but most don't.