Well, if used right (just like anything in TS3) it puts you in the most appropriate position for incoming shot. Which means you really only have to worry when to release a button, rather then worry both, about your positioning relative to the shot you want to pull off.
I have seen people Snap into position while being a good few feet away from the ball and execute a perfect shot.
Glad that is gone now. It made using anything less that a perfectly charged shot useless unless hit into open court.
I can take apart anyone who is not using LB to the fullest in TS3. But my shot variety gets eaten alive by people that do. No matter the spin, no matter the speed of the ball, no matter the depth - holding LB will put you in perfect place to hit the shot. Thus making "variety tennis" useless.