There are P6/P7 features that don't even allow content to be properly used in P4/P5. The practice of putting morphs in .pmd files makes it difficult to use such figures in earlier versions of Poser without a conversion program. P6/P7 nodes do not work in earlier versions of Poser. I just upgraded to Poser 7, but was getting frustrated with how even P5 was no longer being fully supported. The lack of support isn't a DS thing as much as an evolution of where Poser software is going.

Poser has been around close to a decade, so there are a lot more heavy Poser users than DAZ Studio users. Since DS is owned by the biggest posable figure content creator, and is being handed out free, it's going to slowly get more and more support. It's still a new program and it will take time to grow.

The market is going to dictate how much support DS gets. As DS users spend more and more money on products that include native DS support, content creators will have to take notice.  It will probably take a few years before that happens. It will also require DS to become more capable instead of just being a free learning step on the way to Poser. Right now, a proficient DS user has no choice but to upgrade to Poser to get advanced materials, dynamic hair and clothing, and animation.

My visual indexes of Poser content are at http://www.sharecg.com/pf/rgagnon