While there may be little practical difference for the PCs, there's a ton of difference in terms of world-building. For anyone who has no access to magical healing, and doesn't have someone with points in the Heal skill - which is to say, about 90% of the population - it could take a week or longer to get back up to full. Not that it really changes gameplay at all, because PCs do have those things, but it could happen.
For some players, the why of the rule is much more important than the what of the rule. Slower healing might do a mediocre job of representing the natural healing process, but it's still representing the natural healing process. (Or at the very least, it could be used as such.) As compared to the 4E (or 5E) thing with healing surges (and hit dice), where it's not entirely clear what everything represents, and you most certainly can't try to shoehorn that to cover natural healing.