Nut shots hurt even with cups just not as much, your still getting you balls pushed inside you. The point is I guess no one wants to see that or fight in that environment.
My whole theory on dirty fightin negating what most MMA fighters do, is that if all the rules were off MMA fighters would be the best at dirty techniques or avoiding dirty techniques. If anything they would be just that much more deadly and nastier than you average street thug. Sure, I could try to kick Anderson Silva in the groin, but when I miss and he hits me with a flying knee in the balls, 'its all over'.