Difficulty level with autohealing only affects individual battles. What happens in one battle has no bearing on the next battle. However if you have limited healing resources per game level, you have to be careful about expending too much of it in any one battle, and thus may have to rethink your whole approach. Sure, you can still take the careful approach even without incentive from the game, but then you're not really competing against the game, which is kind of the point.
I dunno, viewing a whole map as some kind of complex equation where you have to think carefully about every resource allocation doesn't sound that enjoyable to me.