If you look at reach, the only AA that required the button to be held down was jetpack (I don't acknowledge armor lock as an AA), meaning all other AA's work on a toggle. As a bumper jumper user, I feel sprint works very well on X. All I need to do is tap the button once. It's analogous to jump on A; all other control schemes have you move your right thumb off of the stick to tap A, and you do it very often throughout the game.
So far in Halo 4, every AA works on a toggle (since jetpack hasn't been shown yet) and therefore would work perfectly fine on a face button. I like to reserve the triggers and bumpers for things I want to do while aiming, like shooting, nadeing, jumping and punching. I really don't need to look around to tap a button to sprint.
Agree with all of this, I really want AA/Sprint on face buttons for H4.
Please don't wreck my bumper jumper. I mean, the main things are Jump/Nade on the left and melee/shoot on the right; I can adjust to everything else.