Not wearing a cup - It doesn't pinch your balls, you don't break people's jaws when you go for a triangle, you don't sacrifice technique to use it as leverage in armbars, you're comfy.
Wearing a cup - in the unlikely event of being caught in the balls it protects you, you can tap people with jaw cranks in triangle chokes, you can make peoples arms hurt more in armbars, it pinches your balls, if it does pinch your balls combined with being caught you are in bad pain!
I've never worn a cup, and imo, it's better without. I don't like people wearing cups either, it hurts