Poser 7 has significantly faster render times, and supports much larger numbers of scene elements without having memory problems. Those two features alone make it worth the upgrade, in my opinion. I also have a lot of respect for the multiple processor support, although I can't yet take advantage of it.

As for the new figures that come with it, you certainly don't have to use them or get clothes for them. All of your existing figures and their associated wardrobes should work just fine in P7.