All group member should be expected to contribute what they can and remain consistently engaged. If you are caught up on healing, and don't need a particular utility at the moment, and you have a couple spare GCDs...yes you should be expected to contribute by adding damage instead of twiddling your thumbs. The tank and DPS are expected to be engaged and active 100% of the time that mobs are up- you should too.
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I have never seen a healer blamed for "not doing enough DPS" when there's tons of damage going out. I'm sure it happens, but I think the expectation of healer DPS is not nearly as prevalent or 'enforced' as people seem to think in my experience.