I lick the plate at home. I use my finger in restaurants. It's tacky to have one's face in a plate in public.
Manners are funny things. They're customs that society believes demonstrate respect. When most of society stops believing in a custom, it disappears, and folks who were taught the older ways either drop the custom or get upset because they think the respect is gone as well. This has probably been going on for ages. I remember being taught how to curtsey when I was little. Does anyone ever curtsey anymore except at royalty? How many parents who have little girls have taught them how to curtsey? This also goes for bowing for boys - that thing where you put one arm in front at your belt and the other arm behind you.
As far as I'm concerned, how folks treat other folks is what counts.