Sorry, I have to disagree OP........the reason a lot of times why people are doing a great job killing is because they have an excellent healer with them.
And then there are the times where the players are so uber they simply don't or rarely need heals.....in which case I would be helping killing instead of healing.
In the former the healer is the one who should be getting thanked for heals because if the healer wasn't on top of things the melees would probably be dying instead of killing or killing much slower due to having to self heal.
In the latter I would be contributing to helping with killing, so why would I thank the "killers" either way?