There are many forums for various types of art that don't require people to use the MP at all. People who use PSP, photoshop, those who write or draw/paint etc. They all have forums relating to their art interests that they have to register to post in, but these same people have no need to utilize the MP either. So seeing 400,000 members, is misleading because not all 400,000 members utilize Poser, Vue, Bryce or buy things from the MP.

"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