I think they're trying to force people to upgrade. They did the same thing with Miki and Koji (already on sale in Japan). They have PMD files. But if you save them with external morphs disabled, they work just fine in Poser 5. (Or P4, even.)

This is a mistake, IMO. Maybe if they were DAZ it would work. But they're not DAZ. They're not anywhere near to being DAZ. They're only hurting themselves, and their customers, by doing it this way. Support is everything for a Poser figure, and figures that can only be used by P6 owners are not going to have a lot of support.

I wonder if it would be legal for someone to save Yuki in P5-friendly format, and distribute her RTEncoded? Her page at Content Paradise does not say P6 is required, and some people who don't own P6 have already bought her.