The issue raised relates to the output waveform of the UPS and confusion based on the different options from the UPS manufacturers. There was concern that one type of waveform or another could cause damage if the wrong type was selected. The observation about dirty power was intended as commentary (why worry about nuances in the waveform when the incoming power is a mess?).
There is a legitimate concern about the waveform but the issue is not about potential hardware damage. Some computer power supplies are PFC (power factor correcting), and I am aware that the PSU manufacturers recommend using only a pure sine wave UPS with these. The best I could find in the way of an explanation is this forum citation:
I just spoke with both Silverstone and APC. Silverstone is going to get back to me on this question, but APC was pretty clear -- they strongly felt that PFC power supplies should use true sine wave UPSs. They told me that the reason for this was because PFC power supplies require a much faster changeover time when the battery kicks in, and that due to the design of stepped sine wave UPSs, they were not always fast enough to satisfy the power supply. This could lead to the PSU shutting down when the battery tried to kick in. The guy I spoke with said that the stepped sine wave wouldn't "damage" the PSU, but that it simply might not work, and that if it DID work, it might not "always" work. Apparently it depends on the specific power supply.
This information was related from a UPS manufacturer, so there could be the question of whether they are simply trying to sell more expensive UPSs. However, I have been told the same thing (without the explanation), by a PSU manufacturer.