I've been following this thread with interest but until now I've refrained from commenting; since I don't work with Dawn or Dusk I don't really feel entitled to chime in.
I do have considerable experience with V4, though, so I would like to add my thoughts to this statement.
We mustn't forget that at the time V4 was released she was the best figure available. Her posing was superior to all previous versions and I'm quite sure that it has been one of the reasons for her success. But compared to newer figures her bending is really not very good, as you've pointed out yourself. People still use her because they don't want to switch to Genesis for various reasons and don't want to use any of the other Poser-compatible figure either, also for various reasons. So yes, in V4's case her reasonably pleasing shape combined with a ton of available content (and massive content collections on users' harddrives) outweighs her posing problems.
In my personal opinon Dawn is the only viable alternative to V4. The other female figures all have issues, either in the overall look, in their technical performance or in content support. Dawn 2.0 looks even better than Dawn 1, so this looks really promising, especially for Poser users
However, I also think that the conclusion "nice shape is more important than good posing" is the road to failure. If you end up with a figure that has a nice shape but poses just as badly as V4 you will have gained nothing. Users will simply stick to V4. To make them give up (more or less) on the content they've already collected for V4 the new figure must present huge advantages over V4, otherwise they won't make the switch. So yes, good posing is very important for a figure, especially nowadays where it won't be the only available one but has some competition.
But I'm sure the HW team knows all these things very well
That said, I really don't know why the arms are such a problem in 3D figures. I just checked in a mirror: my shoulders are about as wide as my hips, and when I lower my arms I can feel the inside of my upper arms touch my ribcage but it doesn't feel squished - and it certainly doesn't intersect
I don't know a lot about anatomy but I wonder why this natural movement can't be replicated in 3D figures. They all have issues with their arms lowered, and if you add a bit of heavy morph the issues get worse. And these are serious issues as they interfere a lot with cloth simulations (no simulation engine likes self-intersecting meshes with fabric in between).
I'm very curious to see how Dawn 2.0 will be rigged in the end