There are two ways I could see healers working. Either one might be good for making the game a bit more nuanced.
The first is that they're fast but have low hp, and their "attack" heals rather than damages, so they're only useful on wounded allies. The high speed and frailty would mean they try to dance around combat, attacking as much as possible without getting attacked themselves, and make faster classes useful for harassing them. This would add more strategy to combat, but still make bludgeoning your foes to death generally more useful than fiddling with the ball.
The second is that they're slow but have good hp totals, and again their attack heals. This would mean that they're largely focal points of strength- that is, similar to tanks, you position them near where you need to make a stand, because they're useful but not going anywhere fast. This might allow for strategies that make the game more prone to plans and counterplans, and generally make "how do we get the ball" and similar questions more interesting.
In either case, healers could have a set attack strength like any other class, or they could heal a percentage of a class' max health. I would tend towards the second option, especially if healers are slow but effective, since it maintains hp totals as significant even if you've got a fair amount of healers. On the other hand, this would sort of magnify the futility of attacking tanks vs attacking scouts (and especially of attacking tanks with scouts), and runs the risk of making fast classes not worth healing unless you really need them for something.