CD roms are NOT standard no matter what PC you buy. Nobody is stupid enough to have a 2 years or newer PC and dont buy a DVD ROM. If you can afford a high end graphic card you can afford spending 20 $ on a DVD rom.
Yes many many played UT2003 online. Many played 2004 as well, but not that many because the hardcore 2003 players didnt like that they removed the fast weapon switching. UT99 on the other hand werent that popular because lack of broadband, but this is solved now and broadband is standard in every gamers house.
Even if you dont have broadband access, epic say they focus on singleplayer as well, so there you have it.
BTW : im totally going to play this on my linux platform =) I avoid windows as much as I can because I dont like microsofts idea to keep DirectX hidden from the rest of the world.