I have Poser 5 and 6, and recently upgraded to Pro.  However, I mainly did it because I beta test, and it only made sense to have the more current version.  Though, I haven't installed it yet and am not currently beta testing due to annoying health stuff.

Personally, if the version you are using now meets your needs, I see no reason to upgrade to something newer.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi