It's more of an annoying strategy to play against than an exploit. An Exploit would be like using BXB in Halo 2. This scenario is more like a composition being difficult to deal with and also making the experience crappy for the other side.

That said, I don't see any negative to placing a limit on the number of healers. If your team is squashed when you have 8 healers, they're doing something TERRIBLY wrong or the other team is doing something TERRIBLY right.

In any case, I think the limit should be placed for the sake of preventing enjoyability from going down the drain. If this composition did catch on, the matches would be staler than year-old Honey Comb.