QuickTime X actually supports very few codecs when compared to QuickTime 7, and as such, Snow Leopard includes a full copy of QuickTime 7 (the underlying components) alongside QuickTime X. (What QTX does support though, are the codecs that 95% of people are going to need to play.)
For example, if you play an old Cinepak movie using the new QuickTime X player, it will actually launch a helper process in the background which is really just QuickTime 7. The helper process decodes the video.
Unfortunately, it's unlikely the old codecs like Cinepak, etc. will be ported to QuickTime X - and it's extremely likely Apple will have some reason to remove QuickTime 7 from a future release of OS X.