Well, a vendor can sell dynamic only, if he or she feels that strongly about it, or doesn't want to / can't do the work required for conforming. It's just cutting out most of the Poser market, is all. It's a choice.

Aside from the reasons Gabe mentioned, there is also the fact that most female 3D clothing is, shall we say, rather snug. Dynamic can make beautiful drapes and folds in loose dresses and robes for those willing to  do all the extra steps in the cloth room for each pose. It doesn't work so well on tight clothing, which for better or worse, is how most people like to dress their fantasy girl (if they dress her at all). That predominant situation is where conformers do the best job.... and just happens to suit most Poser users, too.

I wish there were a way to have the dress conform AND have nice draping properties without having to create every drape from scratch in the cloth room. I can make drapes and folds with magnets and even the morph brush, but the former method is clunky in Poser and the latter doesn't do a great job if you're only a casual user of the Morphing Tool.