You can save any time in Donkey Kong 64, but it only saves your play time, number of bannanas, etc., not your exact location and situation within the game. Some games let you save anywhere, even in the middle of a boss battle, letting you come back as often as you like to that exact moment within the battle. This sort of saving can easily be abused.
Well, you know, it's not like it's hard at all to get back to most areas in DK64. Most of the challenge lies in the puzzles, where in most cases it would be pointless to save in a particular spot even if that were possible. But yeah, you're right about those games you mentioned; saving in any spot in a game would be too easy in nearly all cases. But I don't play computer games (except a few online ones), so I wouldn't have even known about that type of game.
Come to think of it, though, in Wario Land II for GBC it is possible to save in any spot, though this might just be a quicksave (I can't recall with certainty). But again, I doubt the game becomes excessively easy because of it, since the game is fairly puzzle-oriented and has lots of secrets to find.
