If you want to just be a decent healer, by all means, just stand around while everyone else keeps doing a role during AK/WP - take a break, watch Netflix, grab a drink.

If you want to be a decent player, you have to do whatever you can to help the group clear content, and that means DPSing 80% of the time in AK/WP (unless you're playing with undergeared or underskilled players) and putting up some DoTs when you can in hard content. Anything less than that and you're not pulling your weight, because I'll tell you what, the tank doesn't stop DPSing once he's established aggro, and as a BLM, if I have the opportunity to throw on Eye for an Eye or Virus, I do it, even though those are mitigation/survival skills rather than my DPS role.