Some reasons why I like having the numpad in its usual position in a full-size keyboard:
-I like having my thumb on the arrow keys while using the numpad, and to have the navigation keys within easy reach without having to toggle numlock.
-My chair's position is dictated by my desk, and my normal configuration of stuff has no space to the left of the keyboard, so the mouse winds up in the same place no matter which side of the keyboard the number pad is on.
-I'm right-handed, and prefer to use the numpad with my dominant hand.
-The placement of long keys on the number pad is more suitable for the right hand.
-Having nothing to the left of the alphanumeric area makes hand placement easier.