Hahaha! I love this @nxtblg!
Ok, I'm also in Ontario and I have personally bought and sold many cars in my day. The reason the dealer was cautioning you is because if you got pulled over by the police without being able to produce proof of insurance, you would be heavily fined and risk demerit points against your license to legally drive. Usually, you have to show proof of insurance before a plate is even issued...that's quite the service the dealer provided.
I would like to suggest that a clear lexan cover for this license plate would be a good investment. You can get these at Canadian Tire. Go for the lexan, it's a little more expensive but it won't yellow or crack as badly as a plastic or carbon-fibre plate protector, over time. This will save your Steemit Plate from stone chips and road-wear as time marches on.
Worth protecting it because when the plate becomes hard to read, you'll have to pay the same $+ inflation to have the plate replaced, otherwise you can be charged for this too...
(Welcome to car ownership in Ontario.) LOL!