I don't know about the technicalities.  But I use M3 because he was the popular male figure when I first bought Poser 5.  I have tried using M2 since then, but have found that there are many more free textures and clothing items for M3 than there is for M2.  Same reasons I use V3 and Aiko.  Although I didn't much care for Aiko when I first got her and didn't really come to appreciate the figure until last year because I really didn't like the "anime look" (I'm not big on anime), but now there are more realistic textures/morphs for her and now  she's my favourite female figure to work with, with V3 falling second.

"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