Thumb-sucking is a problem. It can lead deformed upper teeth by forcing them outwards. And, unfortunately, unlike a dummy/pacifier you can't take it away. Well, not without surgery anyway.
I saw a child wearing one of these devices (above) today. I'd never seen one before and had no idea that such things existed. But they've apparently been around for some time.
But what stops the kiddies sucking the thumb guards??