If JJ2 was even just a little little bit versatile, it would let you make custom levels. Oh wait.
The fact that JJ2 does not allow you to make custom enemies doesn't mean it isn't versatile. Sure, it can't be customized as much as other games, but that's not necessarily a bad thing. And I don't see why you're saying packs like Tomb Rabbit or devres are 'ruined' due to this.