I get down with you because you're down with the X's and O's.
I've always wanted it because it, in my mind, would balance out the defense. Have their mind on two people. The misdirections, putting a man in motion to get the defense unbalanced, etc.
Well, it wouldn't balance out the defense. And that's exactly the beauty of it. If a team has the ability to go both ways with the ball (weakside and strongside, not right and left), it forces the defense into a mismatch by alignment. If they adjust to strength, they leave themselves shorthanded on the weakside. If they don't adjust to strength, they're outnumbered to the strongside.
The real beauty with a multi-back look is that, over time, it starts to create a series of one-on-one matchups out in space. It then usually leads to either a cover-1 (two fly patterns down the sideline create a problem there) or a cover-3 (blocking by WRs or a bunch of short routes will bury that).
I'm not saying Pro-Set being the main formation. Just as a change up to get two playmakers out there and get the defense thinking. Like baseball, sometimes when your ahead in the count, you throw a pitch to set-up a hitter for what your about to throw.
Maybe, have 5 plays the whole game from Pro-Set all runs and then on an obvious run down later in the game set up in Pro-Set, saftey creep up, little playaction BAM! 60 yard bomb. Given that the pass protection was giving that much time for a fake.
Unless it's as basic as a standard split set (TE and flanker one side, split end the other way), there's too much going into it to justify a mere five plays a game.
My last stop where I wasn't running the offense (dumb, I know:blink

involves four or five basic formation groups, and we sucked at every one of them. Understand that putting in a new formation package isn't as simple as adjusting the alignment; there's a ton of work on timing that has to go into it. That's why, all things being equal, a team with a small playbook will usually thump one with a large one. They get a ton more reps on the bread-and-butter plays, as opposed to the inherent question of "How important are these formations and these plays?"
I think I sit and talk with you all day about X's and O's.
Only if you loathe the wing-T as much as I do.