I think there is some misunderstanding here.

Generally, there are two main scenarios that need to be treated differently, imo:

1) Someone in the party is new to the content, undergeared, lagging, underperforming, some combination of these or probably something else is "wrong" with the group (save the "new to the content"). In this scenario, you can't be possibly angry with the healer for not doing dps. I'd actually advice not to dps in this case because it is not worth the risk of wiping. If you really can't help it, go ahead and apply some dots. I think this is what makes good healer a great one - to know when it is the right time to stop dpsing and focus purely on healing, even if it means you have some "spare" time.

2) Everything is going smoothly, healthbars are barely moving, everyone is properly geared and experienced and it is absolutely clear that everyone is on the same page (mostly the older content). In this scenario, it would be very hard to defend the healer who only stands there casting their heals every here and there. This is where I think the criticism is pretty fair and most likely valid.

Of course, there are exceptions to every scenario, therefor it would be wise not to jump on the healer for not dpsing instantly when you start the duty. It is very important to evaluate the situation and then, based on some observations, you can tell what case it is.

You should think twice before calling healers bad for not doing dps. Also, you should not shield yourself forever with the argument "my job is healing, this is what I do, leave me alone", imo. Your job is to contribute to the party. If it was intended for you to only heal, healers wouldn't get new offensive spells in the expansion afterall, right?