Just because an "elite pitcher" does it does not mean that it is an optimal part of an efficient motion. Looking at the mechanics required to get maximum push off the mound you can't get it when the driving foot does not drive but ends up facing second base. How much does it cost the pitcher, who knows. But is certainly detracts from getting the most out of the drive. In one of Hillhouse's Fastpitch TV appearances he shows a student what happens when their push foot is sideways and how you can't push properly.
The driving part is over by then. The foot turns over and drags when they are on the way down