If you hit 2 rebels for 2 damage each, that is 1 or less heal actions worth of damage and as I said, will almost always be a bad play.

If you think it's a lot of damage than play Armored Onslaught and you can Mortar the Rebels every round. It's easier targeting means you will generally be able to hit more figures and makes allies a really bad option for Rebels as you can drop it as your first action on their deployment.

The funny thing is, damage builds up. Sure, it's probably not enough to convince the hero to heal, but it's that much of a step closer to it.

Plus, with ways to deal damage outside of actions (look at Vinto, for instance), an Imp player is going to feel like quite a chump when his damaged units are easily picked off the board without slowing the Rebels in the slightest.

Again, I'm not claiming this strategy is always applicable- but it certainly is some of the time.