I used to play a healer in every other MMORPG I have played before. So, it was natural for me to play a healer in FFXIV as well and started as a Conjurer back then. Well, people started yelling at me for not doing any DPS... when the only thing I wanted to do was to heal. I did not understand why people were angy and so I decided to give up my healing role for a full DPS class.

A few weeks ago, I had the feeling I could give healing another shot and went with Astrologian. I felt like it was way easier to find a good combination of healing and DPSing than it was when I started in January.
Why is it like that? I know the game now, I know when it's ok to stop healing and when there's no risk of letting the tank die.
I did not know anything about it when I played all these dungeons the first time (as a Conjurer/Whitemage). The only thing I could think of was healing, that's it, I was insecure and afraid of making mistakes.

So, what is it what I want to say?
If you don't feel like dpsing as a healer, don't do it. It's not neccessary in any normal dungeon. But if you want to, go ahead, there's no problem with doing that
I don't know how it is in raids and stuff, I'm no raider and never will be, so I can only talk about "normal" content.