The combination of skeletal structure, musculature, and fat do a very good job of making individual faces very unique. We meet thousands of people in our lifetimes, but find very few that look alike. We may all have similar facial muscles, but we don't all have the same smile.

Poser expression morphs aren't moving 3D muscles, but are moving vertices that don't have to slide over anything that preserves the overall shape of a celebrity likeness morphed head. It would take a lot of effort to create a Jessica Alba likeness and make it still look like Jessica Alba when she's smiling. The odds are pretty good that new morphs would have to be added to do the job.

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