That's not the problem. She *can* and wants to heal in melee, but raid mechanics prevent her from doing so. On the Bladelord boss in HoF, second boss, he has a wind strike that will hit "ranged," putting a very damaging debuff on them and anyone nearby. Were she to stand in melee with the rest, it could hit her, the tanks, and all melee gathered up. Mistwalkers get to avoid it, as they don't count as ranged (Really? That's cool to know!).
But paladins, which have the ability to melee for mana (and should, if you're good enough to handle it without losing focus), are told we can't. Meleeing for mana is our Power Word Solace, Telluric Currents, or Wrath of Cenarius (Need to check if that thing still exists, haven't looked at Druids since Cata). And it's a kind of discrepancy. Not a huge one, but still something that is off.
As to your question OP, I stay in ranged full-time. Earlier in the expansion I was given the excuse that too many HoT's rolling from eternal flame prevented me from doing so efficiently, but the truth is I simply prefer the hang-bank approach. Not being clustered up in melee gives me a brighter view of the battlefield, and concentrating on melee for mana optimization takes away some of my reaction speed. I tend to precast and cast continually anyways, so there's not much reward for the sacrifice. <thinks about the bosses> There are many fights anyways where dps and tanks are spread out, so I have to position myself in a central spot that prohibits meleeing. I do melee when I can get away with it (Feng, Elegon, etc.), but it's few and far between.
Edited by Wards on 3/11/2013 2:27 PM PDT