It's unlimited, but within a specified range. Since we know that it's always a physical injury (or we know that it's always a mojo injury), we can be more detailed in how we treat it in terms of interaction with other things.
Can you provide an example of this sort of interactions? (Other than healing times?)
If we know that every HP contains a non-negligible meat component, then we know that healing must be consistent with physically repairing the body.
Which brings us back to the point that most people in the gameworld (including low-level PCs) never suffer injuries that are not potentially fatal yet take more than a few days to heal. Nor do they ever suffer injuries that are incapable of healing without medical or nursing care.
To me, that doesn't make for a consistent gameworld. Are these people, or mutants?
If we know that every HP is entirely mojo, then we can heal everything with inspiration (or one night of rest) without fear of contradiction.
If you keep everything abstract the entire time, and we can't guarantee that any given hit was actually a hit or a miss, then we can't guarantee that it's appropriate for any given injury to be healed with inspiration (or overnight).
You are missing the option that in fact I, and other posters on this thread, use. I pointed to it quite a way upthread, when I pointed out the distinction between healing in the sense of hit point recovery, and healing in the sense of physical restoration of the body.
Hit point loss, and hit point recovery, in my view correlate to losting or gaining the ability to fight on and win. Some
events of hit point loss might correspond to (non-serious) physical injury (eg scratches, nicks, brusing, of the sort Gygax describes). But the restoration of those hit points doesn't mean that the injury is healed. It just means that the injury is no longer a burden on the PC's ability to fight and win.
In Gygax's AD&D, hit point loss that drops a character to 0 hp or below is different, in so far as it corresponds to an event of suffering a more serious injury (immediately fatal, if you drop to -4 or below, and potentially fatal if you drop to 0 to -3 and then bleed out to -10).
In 4e, hit point loss that drops you to -ve bloodied is different in the same way - it correlates to an event of suffering a fatal wound. Hit point loss that drops to zero or below, and that is then followed by 3 failed death saves, has the same character.
Even in these cases, though,
recovering the lost hit points doesn't correlate to healing the serious injury. In AD&D that takes a week of rest or a Heal spell or equivalent, regardless of hit points restored (DMG p 82). In 4e, if a character is inspired or roused back into action, that tells us that the injury wasn't so serious after all.
In neither case is the hit point recovery is not correlated with physical restoration of an injury.