I guess that the majority of new computers *DO* come with a UPS, as the majority of PCs now sold are either laptops or tablets.

However, all of our HP servers have a small L-Ion battery on the disk controller, to retain unwritten data in the event of a power-out, despite the fact that the entire server room is on a UPS (you can never be too sure) and it should not be hard to include that in a PC's M/Board design, using a small button-sized battery.

Nevertheless, a good quality PS should be able to withstand momentary interruptions to the supply, as most quality PS's have at least a limited capacitance, but there are no hard & fast rules that allow you to spot the 'quality' of the PS in a PC - it tends to be hit & miss (so to speak).

At the end of the day, for protecting running applications, there is no substitute for a decent UPS, and we have them scattered about our offices, for all desktops and terminals - sometimes with multiple devices on the one UPS - because the supply in our area is notoriously unreliable, in outage, voltage and spikes.