Unlocking the t-spine, allowing it to rotate and improving extension will help to correct a variety of shoulder, elbow and even low back issues. However, in order for the t-spine to move optimally, the muscles securing the pelvis and lumbar spine must be working accordingly. The consequence of weak LPHC stabilizers results much like trying to open a soapy pickle jar. If you were grasp the jar with one hand and attempt to turn the lid with the other, you wouldn't be able to create enough torque to unscrew the lid (the jar would just slide in your hand). If your LPHC isn't stable (hand/jar) the t-spine (lid) can't rotate on its own, which in effect inhibits core stability, and impairs dissociation of the hips and shoulders, leading to a chain of problems.
This principle doesn't just apply to exercises designed to mobilize the t-spine. Every single mobilization exercise requires active, dynamic stabilization. Even the simplest glute activation exercises require the torso to be stabilized in order to ensure that motion is occuring at the hip and not the spine.