I get the intent of a convertible crib that can be used as a full size bed, especially for people who want to buy one very nice matching set of furniture and have it last for 18 years.
But if you're not going to do that, I don't get it. I can't imagine putting a 3 year old in a full size bed, nor can I imagine having room for a large bed in a child's room full of toys, play kitchen, etc. I would not put a full size bed in my guest room, I'd rather have a queen in there for adult guests, so I see no use for a full until the child is 12. And a twin headboard is not that expensive, so it's not like you're saving tons of money by buying a convertible crib.