You can't have realistic shoulders without JCMs anyway, and with the arms bent down to 45°, you need two of them, one for each end-point.

With the arms stretched out in a T-pose, you only need one as the upper end-point is already the default pose.

Same is true for "crouched" legs and "ballerina feet". (Which make rigging flat shoes a nuisance)

I also find a figure where the default are fully stretched limbs much easier to pose because a dial value of 0 is the same as a 0°-movement of the limb. Without an extra JCM, a "pre-bend" joint will also never look perfectly right when straightened.

Again, Poser meshes are different from professional Studio meshes.

They need to be photorealistically accurate, very flexible, easy to use and easy to create add-ons for. Ease of animation is pretty much unimportant.