Part of the problem here to me would seem to be the fact that people assume that software is shipped in a finished state and bug free when they purchase it.

As poser has become more complex and later versions had more bugs that showed a bit more dramatically.  Most people don't have a clue they should go to a web site to check for updates to a program, much less the ability to actually find and install the updates.

Honestly any version of poser runs relatively stable on my computer.  P7 does appear to use quite a bit more resources than P6, but that's what happens as a program gets more complex.  I didn't see enough of a difference between P6 and P7 to justify paying the price to upgrade, but that's my personal opinion.

As far as the interfaces go I haven't played with Poser 4 in a very long time, but the Poser 6 and 7 interfaces seem almost identical to Poser 4 to me.  The only major differences that stand out in my mind are the choice of render engines, the shader nodes in the material room, and the inclusion of the face room, materials, setup, and so on.

Those extra tabs aren't exactly hard to figure out.