When you bought the plates, was there not a little sticker saying for show use only....or a comment made on the website you bought them off (assuming you bought them off line). You could try pleading guilty to the plates but explain you were unaware of the fact they were illegal until you got pulled up....also assuming this was your first pulling for the plates. Pleading guilty means you accept you were in the wrong but wish to put your self at the mercy of the courts. Also it shows you are not trying to undermine the policing methods of the state.