Not everyone wears them, no. But the vast majority do - I'd say 4 out of 5 wore them in my Mods, last term. It's quite a nice tradition - even if you're not superstitious about them bringing luck, it's fun.
It's supposed to be bad luck to buy your own. In my college, our college sisters (our equivalent of college parents) buy them for us, and pidge to us the morning of the exams.