I hate trying to start posts. I want to just repeat my title. It feels like a good first sentence. It gets to the point and doesn't ramble, unlike everything else I write.
So yes, why would I want this?
Sometimes you just don't need a big heal. Sometimes even flash heal is too big.
This thought was brought on by a recent SFK run on my druid. A 41 shaman was running a warlock and I tagged along. It was pretty slow, partially because the shaman wasn't very good (strange spec, strange gear, and just slow moving). He kept telling me to heal because "I'm the weakest one." That seemed strange, since there wasn't much to heal. I thought, why not have him heal, I'd pull a bunch and tank it while the warlock was AoE. That sounded fast. But the shaman didn't seem interested.
In retrospect it wouldn't have worked well. The overhealing would have been massive and as a result the shaman might have been unable to keep up. Downranking would have helped then and I think it would help a lot now.
What's the nerf?
Downranking was removed, at least as far as I know, because it was giving ridiculous efficiencies. Lower ranks gained less from +healing, but not enough to offset the lower mana costs. So why not have the total healing reduced directly in proportion to the lower mana cost? This would mean no gain in efficiency. Instead healers could use this to reduce healing output for a lower mana cost. Currently this is possible, the it either involves changing spells or delaying casts. Neither is very good.
Changing spells, for example HL to FoL, will result in much lower spending but also much lower output. This is frying pan to fire: massive overhealing to massive underhealing (also known as a dead tank). Mixing would reduce the effect, such as alternating FoL and HL, but then this just means sometimes you underheal and sometimes you overheal. With luck the tank won't die from the underhealing times, but luck isn't always nice.
Delaying casts is even worse. Tanks tend to need healing now, or at least 1.5 seconds from now. Delaying heals means they don't get that heal. Even if the healing averages out properly, tanks don't live by average; they die by extremes.
Downranking would also allow for more flexibility. Do you want to skimp on regen, stack SP, and downrank? Or do you want to go for more regen and use higher rank heals? Maybe go somewhere in the middle and balance the two with a fairly high rank. This would also help to make Ulduar gear (at least as I remember it a couple weeks ago) more attractive. Downranking makes crit less effective compared to mana/5 and allows for an actual choice between the two rather than just crit>mana/5.
In the reverse direction, how about talents with effects like "increases healing by 50% and mana costs by 50%, lasts X seconds." Bad things are happening and you can respond to them, but this isn't something to spam. Or maybe it allows regen to be an output stat as well since it would allow a player to use this cooldown more often.
Maybe this is just the wishful thinking of a bad healer who wishes the game was more accommodating of his lack of skill.