We don't know what the second Song button is for. I think it's a little too premature to say if it's needed or not. My educated guess is that it has something to do with saving/loading actual Song files (that you made in Song mode) from your hard drive. Maybe it brings up a list of all your save song files? Maybe in .sng format or something? Although, if this is what it's for, it could be made as a shift function for the Song Mode button. But they probably separated it to keep Data function buttons away from Mode function buttons. My guess!
I personally think the shift button should stay where it's at. The majority of users are going to be teenage to adult men. They will have no problem reaching from shit to function button with one hand. However, if you move it, people with huge hands are gonna have a hard time pressing shift and accessing the Sample or Song button for example, because you'd have to twist your wrist in an uncomfortable way to hit them with your index or middle fingers due to the buttons being laid out horizontally. You'd likely have to reach those buttons with your pinky, which would also be uncomfortable after several presses.
The reason the close button layout works on an XL is because the buttons are laid out in a more vertical fashion, which is how your hand spreads out in relation to the buttons when sitting in front of an MPC. Our hands don't spread sideways, which is what they would have to do if the shift button is to close. I'd have to turn the MPC sideways like the picture below to use it without spraining my wrist.
One more time for reference:
