Actually, healing, in low-op groups that don't wandwhip, tends to go to most in danger of getting knocked out/dying first. Or, if the DM isn't doing his job, the 'tank(s)'.
If the wizard gets hit with a longsword for 7 damage, and the barbarian gets hit 3 times with a longsword for 21 total damage, one spell could conceivably heal the barbarian to full while not healing the wizard all the way. That's not 'the same', that's 3 longsword hits healed vs 1 longsword hit healed, with the same spell.
Casts to incoming damage is what i'm talking about. It's vastly more efficient to heal the higher hp pool people than the lower hp pool people, assuming that damage isn't a % of health as well (which I don't think is being proposed here). So it doesn't make tactical sense, under a % system, to heal the squishy caster. And if you have to, you'll feel like a chump for doing so, because you're 'wasting' heals that could be wiping out 3-4 longswords worth of damage in a single cast on a guy who's only taken 1 longsword hit, and the cure doesn't even heal that up completely.
If the wizard has taken 2 longsword hits and is on 40% health, and the barbarian has taken 5 longsword hits and is on 60% health, the barb is going to get the heals and the wizard is going to go begging. It just encourages MMO-style 'tanking' and whatever for the DM, because the cleric won't want to 'waste' heals on the wizard.