I see those covers on plates once in a while here but yeah, they're illegal and might get you some unwanted attention the rest of the time.
The move around here is to drive a new looking car and just never install the plates. Leave the little piece of paper from the dealer on the windshield.
One of my exes got pulled over a long time ago in her one-year-old Benz w no plates. Cop asked, "where are the plates?"
"In the trunk."
"You need to put them on."
"OK. Do you have a screwdriver?"
Doesn't work if you drive an old hooptie, though.