If a new UT3 came out with just the new content you would have to have it cost full-prize again (because it includes the full game) and just the patches aren't really worth buying the game again if you already have it.
Or you can make it only cost what the expansion is worth, but then people who don't have it yet can get it for (very) cheap.
So neither way is "ideal" for UT3.
What would be ideal would be to just release it as free dlc/patch, but I understand MS and Epic were never able to find a way of doing this that suited both parties.
I don't know why Epic wouldn't just charge 400 ms points for it or whatever updates of this magnitude cost elsewhere. It wouldn't be as nice as getting it for free, but it would be better than nothing.
Halo 3 and ODST are two different, moderately long campaigns and they only share the multiplayer component, as far as I understood it. So no matter which one you buy first, you can buy the other one afterwards too without feeling cheated (provided you like the series and you are interested in the campaigns).