You should be able to use your efficient spell virtually forever (or at least much longer than the average encounter length). That efficient heal may be fine for earlier dungeons or raids. Eventually though you're going to hit points where someone is going to die in between casts of the efficient heal. In those cases you should switch to an inefficient but fast or big heal. Yet, if you focus on the inefficient heals too much, you will run out of mana.
As players gain more and more regen in later tiers of content, they'll need to rely on their inefficient heals more, but will also have the stats to be able to do so.
Not every healer has enough spells to currently make the decisions between efficiency and other choices. In fact, we'll probably have to adjust the toolbox of everyone to some extent.
[...] Most healers spam one heal now. Druids for all of their spells can do very well with just Rejuv and Wild Growth. Disc priests spam PW:S and Holy priests spam CoH on cooldown. We want to carve off niches for other spells. Flash Heal can be awesome if it runs you out of mana to hit nothing but Flash Heal.
[...] Casting a couple of fast / expensive heals to save someone's life is exactly what you should be doing. Casting nothing but the fast / expensive heal because, hey why not, leads to pretty repetitive gameplay.
[...] We will have to change numbers for Cataclysm. It's probably a safe assumption that we'll adjust every number in the game in order to account for changes such as larger health pools, lower mana regen and all of the talent tree changes.
This sounds great... kind of. What I mean is that it sounds great provided that they realize that in many cases the fast, powerful spells we spam these days are *also* the most, or among the most, mana efficient spells in our arsenals. I've gone over PW:S before, but when they pull out the examples of Rejuvenation and Wild Growth it gives me a facial tick. I think they know - I really hope they know - that druids use these spells almost exclusively because they are better than other spells in nearly every conceivable way, not because they have too much mana.