@Angus

'Which is what the original intent of the healing changes was. It was supposed to be "heal as needed, use huge or fast heal as needed and THOSE will mana drain you if you spam them.'

From what I recall (and I don't believe I have seen someone at Blizzard state that you should NEVER run OOM using a standard heal), it wasn't that you were "supposed to be able to never run OOM" if you used your standard heal, but it was the most efficient of all the heals with the drawback that it is not suited for specific tasks. Fast heal if you need to bring a target up immediately or a big heal to... heal alot. Spamming any 3 spells constantly will run you OOM or overheal. The playstyle they want to ram home in Cata is that healers will have to find efficient uses for heals in respect to their mana.

'So instead of healers being able to moderate the drain rate it is a clock.'

You basically always have, and probably always will have a 'clock' counting down to when you'll be OOM and that timer has always been in the control of the player. It's up to you for what type of heal you use for specific situations which will govern whether you will run OOM in 8 minutes time or 3 minutes time. If you spam fast heals or big heals, you will severely drain your mana pool within a short amount of time. If you spam your 'standard' heal, then you will last longer but the risk of others dieing due to certain situations may increase. If you mix in all 3 in reaction to situations, you'll find a sweet spot in which you'll have enough mana to last you while keeping everyone alive.

'So why is it okay for a healer to run out with the basic heal?'

It's what's going to make healing a bit more interesting. As GC has stated, is a bit more beyond 1 - 2 button healing. Nevermind now having an actual risk of going OOM due to poor choice of actions when it comes to healing.

Personally I've had to heal through BC and WotLK and this direction does interest me, aswell as others that I've talked to ingame that mainly heal. Though I tend to enjoy the 'busy' playstyle (I like playing as feral cat :P )

If you want to read more to understand, try this post from GC

http://forums.worldofwarcraft.com/thread.html?sid=1&topicId=21627632910&pageNo=1#6

He has also posted a few 5 or so more times in that thread relating to priest healing, downranking and regen.