I do my own posing, and a true T-Pose figure makes that much easier because you know if the bending is "0", the joints are perfectly straight.

With pre-posed figures, I'm never really sure what the true default shape of the joint is.

I guess it's not that important with stylized figures that see a lot of "action" because movement hides a lot of anatomical flaws, but for the precise sculpting and static poses I use, the true T-Pose allows much more accuracy.