Were you asking the question where a real situation presents you with a real assailant who is wearing a cup? Or a training scenario?
If it's a real situation your still going to get his attention if you slam him in the groin, cup or not it still hurts. Instinctively, most guys will protect their groin with their hands, human nature I guess. Well, that leaves his head wide open......good for elbows.
Krav is all about options and having multiple methods of attack for each body part. Even if his groin is protected, chances are his noggin, knees, solar plexus, kidneys, eyes etc etc etc are not.
Have fun in your training!