Well if you get caught in a National Forrest or a State run riding area you could get fined and told to load up and leave till you get a forrestry approved spark arrestor put back on, If you are not racing leave it in loud pipes get areas closed down.
Yep its one sure fire way of getting kicked out of the places you ride. And it makes such a minimal difference theres just no point. If anything buy and aftermarket slip on. That will give you some sound and power( sadly you will need to rejet tho)