The rule of approval is quite great since a single 8 star approval map is really scary. Support this!
But it's quite weird to restrict the amount of difficulties in one level. I guess the entire rule is made here to make all levels pf player could enjoy the game more. Since the style among mappers varies, it's not bad, or rather, better to have multiple difficulties within one level, because the variety of mapping style caters on the need of all kinds of players (Different players enjoy and are good at different beatmap styles.)
But it's quite weird to restrict the amount of difficulties in one level. I guess the entire rule is made here to make all levels pf player could enjoy the game more. Since the style among mappers varies, it's not bad, or rather, better to have multiple difficulties within one level, because the variety of mapping style caters on the need of all kinds of players (Different players enjoy and are good at different beatmap styles.)