My 2 cents.
Starts with a question??
What is the industry standard???
Poser should stick to the industry standard.

Building and rigging is easy with a standard "T" pose.
During all my Poser years, back from Poser 1, I never needed a JCM.

I rig, adapt, morph, re-rig, morph, re-rig, morph, re-rig, untill I get a bood bend with conventional rigging.

Now with WMapping, the process is speeded up quite a bit. I need to make far less corrections to get a bend right.
Some of you have seen my results. => Teyon.

But???
There is another matter that matters more.
Uv-map streching.

When you build and rig and uv-map in a "T" pose you have a problem at the shoulder bend and at the elbow bend.

When pulling the arms down, or when bending the elbow, the outer surface get streched a lot, and the uv-map streches loosing all detail in those area's.

For the uvmap it would indeed be better to build and rig with the arms at 45° down.
But ?? Then you also have to build and rig with the forearms bend at 45°.
Building with the forearms bend at 90° would even be better for the outer elbow uv-map.

I have come to good results with conventional rigging and with WMapping to rig without any hidden thing.
No hidden bones, no JCM's, no hidden magnets.

The only problem that is left is that uv-map streching.

Best regards, have a nice day.
Tony

Poser 1, 2, 3, 4, 5, 7, P8 and PPro2010, P9 and PP2012, P10 and PP2014 Game Dev
"Do not drive faster then your angel can fly"!