Just putting my two senses here:

A healer's dps could mean the life or a wipe of an entire party. It DOES make a difference, even if it's just a few hits here and there with re-applying their DoT (or DoTs). I always dps when no one needs healing and/or is topped off. I only switch to pure healer is when there's a lot party-wide damage going out. But most of the time I'm in dps mode with the rest of the party. People need to get a bit out of their comfort zone and just dps. Dps when you find open windows. It makes a difference in a run with it bit being faster. So please, healers, dps when you find open windows.

Signed: a fellow casual healer.