What a silly rule, you can't use them internationaly anyway.
Kayty - In GB tack stewards are super strict and apart from boots you are not allowed to warm up in any tack that you will not be riding your test in. Doubles only allowed at medium and above, but snaffles allowed to GP.
I will say in showing I have used earplugs but showing is a whole different ball game and they are allowed, when you've got an evening preformance with 20 horses cantering in a group, an electric atmosphere, music, cheering (right the way through the go round) and a somewhat explosive horse it is the safe decision, that said though on most horses I get them used to it before hand and go without.
I've also used them at a major county show where my ring was right next to the fairground (as in one of the rides extended out over the ring). I popped ear plugs in too prevent spooking at sudden screams etc.