The main issue is quality, QT just doesn't render as well as Cleaner at similar bit-rates, although it is often a lot easier to export from an iApp via QT to MPEG2 than export to DV and use Cleaner.
I have the new Compressor app that comes with FCP4, but haven't used it yet.
iDVD uses the QT codec in the background as you import the clips, but seems to make a decent job of the encoding.
Incidentally, if you collate the clips in iMovie, you can bounce the whole thing directly to iDVD using the "iDVD" button, so you only need to worry about getting the clips into iMovie. Be aware there is currently a limit to the amount of video you can use in iDVD, 60 mins I think.