iNITIALLY I didn't wear one (because I hadn't found one that fit yet) and then one day got knee slice passed and also had someone almost awkwardly land near the groin so I went out that next day and ordered a Shock Doctor cup online. Since then I've purchased 2 more and those are the brands I wear, I've never had anyone complain that my cup was the reason they tapped (I also have almost never had it be the reason I tap, in the few cases I could feel the cup against my arm or neck the lock was just about sank in anyways and it was going to be a tap cup or not). I treat cups the same way I treat seat belts, helmets, condoms, etc.
All things being equal you should use them, they may be uncomfortable at times but in that moment when you need them you'll be glad (I've had a couple kicks in TKD come near me, and in rolling too someone blatantly land near my crotch and thanks to the Cup I was able to continue going like nothing happened).