I think the point is less about magic being inexhaustible, and more about whether running out of power is fun.

Most healers didn't pick their role because they found the bottom of the barrel exciting. They did it because they enjoy the contest with the enemy over the lives of their companions. "Can I protect and support my team in the face of their assault?" Reaching the point of OOM-ness is more of a feeling of "well, we're done. Time to sit back and die." The fight's not even over, but it's over. Frustrating.

I'd really prefer an active system. It has the same preference of efficient vs. powerful, since using the big heals will deplete your resources and make you less able to react NOW, not 5 minutes in the future when you run out 30 seconds ahead of schedule. So, yes, you still have to pick the right heal for the job.

Consider also, that the better your gear, the more mana you have. Instead of reinforcing the efficiency situation, it makes it easier to throw out your less efficient heals. Regen scales faster than efficient heals.

Active resource means you have to be aware of the fight, know when the big damage is coming and be ready for it, and have enough personal power to handle it when it comes. Isn't that what healing is really about?