We enquired about these when we first started looking at the weddings at Disney.
When we went for our site visit and asked about them, they are aparently quite rare, but if you did have them, they can do whatever they want; they can 'pick' on someone of your choice if you know they will get a good reaction, they go around and chat and mingle with all the guests. You can even hire a 'clumsy waiter', who will go around and almost drop things all over your guests. We was told that those who did have them at their weddings went down like a house on fire and really livened up the atmosphere.
However we decided we have enough characters at our wedding and chose another form of entertainment.
Hope this helps!