FWIW, I agree its better to avoid altering higher level files... If you're only using 1 window that makes use of those styles, or a handfull of windows in the same UI branch with a relatively simple style set.
The overhead of including a large style set into several windows might cause a noticable performance hit. Someone with better diag tools than I would have to test this. Not sure if EQ2 would end up loading the same resources multiple times due to different window nestings. If Death chimes in on this one, I'd trust him. He's probably more of a programmer than me
Also haven't tested if its even possible to span UI branches with style refs. My latest Journal mod includes the HTML window, which isn't inside the Journal branch.
As far as not wanting to customize eq2ui.xml goes... in the 3 months I've been including eq2ui.xml, I think its been changed by SOE twice, to add SplitterStyles and IME. Its been the least of my troubles
