Scaling it to the hit points rather than level of the recipient would seem to make the most sense. Which is what 4e does, as every healing surge or equivalent restores 1/4 of the hit points of the target.
Well... Not really. Some hit points -- the ones based on larger HD and Con bonuses -- DO represent the ability to withstand or endure greater injury (which will then take longer to heal).
It's the recovery of hit points, particularly magically, that seems to me the point where hit points stop being a sensible way to handle conbat resolution and become something rather peculiar. If you're really not taking much physical injury, instead becoming slower/unlucky/feebler/cursed, then what does magical healing restore? Well, apparently it's "dissociated".
Well, to play devil's advocate, one could say that the spells heal a number of
specific wounds -- paper cut or gaping gash in your side, the spell doesn't care. (Magic is finicky that way.) And since those with more hit points have, in fact, suffered a larger number of less severe wounds (by turning each blow that lands into a less severe wound), it requires progressively more powerful magic to close up the progressively larger number of wounds.
But, yeah. I generally agree that the
cure spells are not tightly associated with the game world.
However,
cure spells are still more associated than healing surges (which have all the same problems of abstraction, but also tack on a strange limit on the amount of healing you can receive in a day) and abilities that allow you to physical wounds by yelling at people (only slight hyperbole there).
They're also, IMO, better game design than
the hard limits of healing surges. I'm not a big fan of the 15-minute adventuring day, and it completely baffles me that the designers of 4E included a mechanic which, for the first time ever,
mandates a short adventuring day.
But here we begin to go a bit further afield than the original topic.