People who run high level content and actually want to win that content care quite a bit about how much their teammates contribute to that goal.
I know that both you and the poster below you are claiming that healer damage is negligible, but it is not. It varies by healer class, but in a good group it can be around 20%-30% of one of the DPS players, the same as a global 5%-10% boost (or 5-10 ilvls) to all three of your DPS players. Do you consider that “nothing?” Because I sure don’t.