• The capability to join the rest pose from one/multiple (source) armatures as keyframe pose/poses to another (target) armature.
• The option to lock the scale or orientation of the target bones.

Two method for transferring bone transforms to choose from:
Method is the method.

• "Use Matrices" (Recommended). Original method from Join as Pose 1.0 Release transfers exact rest poses from the source armatures. Might be a bit slow for huge skeletons (1000 bones or so) and multiple source objects.
• "Use Constraints" (Faster) transfers current poses from the source armature (NOT THE REST). All constraints from the target skeleton (active object) will be cleared. Should be considered if you have a huge armature and need to join a lot of source armatures, or need to transfer transforms from the current pose, not from the rest one.