True story from HQ about the importance of cups. There was a young man who never wore a cup. I repeatedly told him to wear a cup, at least six times over the year or two he was with us. Then one day someone went hard with a knee slice and smashed his testicles. They took him to the hospital where he had half a testicle removed because it was smashed to pulp. I've been through several serious, by jiu-jitsu standards, injuries including broken ribs and a torn meniscus that required surgery, and returned without thinking twice. My balls, however, I prefer not to endanger.
Some major tournaments make cups illegal, including Gracie Worlds and others. The argument is usually the cup makes finishing a triangle easier. It is true, sometimes you tap not to the triangle but to the cup. I'd rather we all took that risk than the other risk.
I have done no-cup tournaments and it scares the hell out of me. It changes the way you roll. I like playing half guard but without a cup you quickly realize that you have just put your opponent's leg in a place where he can do damage. I don't even like to roll at the gym without a cup. I'll hazard rolling without a mouth guard, but without a cup - very rarely. Nine out of ten times, I'd go home (although now i keep a spare cup in the locker, just in case).
So if you train with a cup and want to compete in a no-cup tournament, at least a month beforehand take the cup off to acclimate yourself to rolling that way. I think that not doing so hurt me when I did Worlds. It really bothered me during competition, especially once I pulled half guard.