Well, there is a notable difference between movies like The Little Mermaid (G), The Nightmare Before Christmas (PG), and Spider-Man (PG-13).
It's arguably easy to see the more "advanced" concepts that exist in TNBC vs The Little Mermaid (scarier material, jokes based on death, Santa Claus threatened). Similarly, Spider-Man has much more violence, danger and more "adult" dialog than TNBC.
So I don't think the PG-13 rating is useless, I just think it's not being used the way it should. It seems arbitrarily assigned based on the "type" of movie instead of it's actual content. It's all just become part of the marketing sceme.