I would love to see this happen, however, I don't know if the risk of injuries would become a major issue or not. Considering that you normally 'shouldn't' be able to touch the QB, and overall, would expect each player to play a bit softer to avoid this unfortunate occurrence, would it be worth watching?
Imagine two Power 5 teams meeting in a spring scrimmage... The fans would want a competitive battle, one that you can brag about and perhaps take something from, but that wouldn't be possible when each team is set to standards, special rules, and restrictions.
On the other side, if it's a Power 5 against a lesser FBS/FCS team, then what's the point then? Another showing of cupcake dominance? Meh.
But what if the game became competitive, as most would expect to see, and perhaps, it got out of hand, which is what I could see happening between two major programs.
It would be fun, and it would momentarily sate my appetite for football during the off season, but overall, I don't see it working out.