I was DD for a wedding last year and when I got back to the car I parked on the street of a friend's house it was egged. It washed off and while it was a pain I'd rather than then have the car keyed since it didn't cost me anything.
Our car was egged once in the middle of the night or early hours (along with a few of our neighbors' cars) and it ruined the paint on the new car. Raw eggs will eat your paint-- I know that from experience. Maybe if you wash it off soon enough it doesn't, but we weren't that lucky.
People who do this are losers! Especially when it's done to strangers for no reason.