Posture has a whole lot to do with it as well. 

It's important to get a figure's posture and proportions right and try to keep them as balanced as possible so that the the modeling process is less of a chore and less prone for making mistakes. Different features can be difficult to model accurately if the center of balance is too far off from what a real human would have in the same pose. 

It also creates problems in rigging, since the rig is being built based on the angles and twists of the geometry in its default shape. If you have to twist the thigh  15 or 30 degrees from its default position just to get it to align properly with the rest of the body the way a real human's thigh would align if s/he were standing in the same pose, then how well is the figure going to bend that knee or hip? And how off is the torso going to look in proportion, since thighs and knees and various other bits are used constantly to guage where the placement and proportions of other parts of the anatomy are going to be or should be. 

I understand the bit about the T-pose being required in order for walk designer to work, but the rest of the posture shouldn't be affected by that. And if it is then walk designer needs to be redesigned so that the limits it places on figure modeling and rigging aren't keeping the figure from looking like an actual human. 

Bad posture can throw off proportions and details through the whole figure, and it doesn't take a whole lot of change to make everything else a challenge and look just a bit or a lot strange. 

 

~Shane