We came from the ITF, and wear white dobaks for the reason upnorthkyosa gave, and also because it helps to create a mindset - this is a time and place for TKD, not for the world to intrude, and having everyone dress alike (except for belts) helps with this.
KK1975, I don't know anything about a requirement for t-shirts, and I was in the USTF/ITF for a long time; t-shirts in the USTF were up to the instructor in class, and were never required for anyone, although they had to be plain white at events (testings, tournaments, etc.). I avoided t-shirts (I get heatsick fairly easily, and the extra layer just made it worse) and avoided having my top fall open by sewing velcro where the top crossed itself. During the summer, we wore t-shirts to work out (no AC, in an upstairs room with poor ventilation), but there were no real rules about what could be on them, as long as they weren't obscene, and I wore tank tops then.