I think I prefer the changing parties. I like it when characters leave the party for a story-based reason and it leaves you wondering whether they'll be back (and if you'll ever get your stuff back >_<)

The underleveling of late joining, frequently leaving, or just plain unliked members of the party isn't really an issue for me. It doesn't take long to get them back up to speed if you need them anyway.

edit: I just voted for static accidentally. =(