Thing is, it's several orders of magnitude easier to map for Doom than for UT3. Even taking into account map balance and strategic placement of items, a program can generate a blueprint for Doom relatively easily.
UT3 adds several dimensions to the mix... one of them literally (and good z-axis play can make a great map!). A BSP generator that doesn't give a crap about visuals and uses a collection of, say, 6 materials could be technically feasible, but most of its maps would be utter crap to actually play on.
I can see how some of its output would be usable, though. Not 'playable' in a usual sense, but usable as a baseline for a real map.