We kept it out of the tool specifically because we don't have the resources to test all the different possible resolutions people would want us to support through the entire game for each resolution. The choice was: A) Ship a few resolutions, but all of them supported 100% or B) Ship the game supporting every resolution under the sun and let the game look crappy to some of the people running it. With these things in mind, it was pretty easy to go with A and support option B quietly by allowing these ini values to be manually edited.
Well, 1920x1080 (1080p) is pretty common these days due to it also being the standard in most monitors and HDTV's these days. That resolution should have been tested.
I would wager that 1920x1080 (1080p) is more common than 1280x1024.