It works great for storage, but you have to remember the trucks beginning and general use - hauling things in the bed - most people want the space inside the bed, rather than giving it up to storage. Under the seat is a good option.One of the coolest things I've seen in a pickup is the side storage compartments on my brothers Ram truck. Surprised more truck manufacturers don't do that. He's also got a storage area under the rear seats. The bottom of the seats lift up really easy and you can stow whatever you want so that when you put the seat back down they're out of view. Outside the 12mpg and 0-60 in forever issues, it's a really solid truck.
Very few people know this, but the Ford Supercrew rear seats will fold down from the back. It's not meant to be done on a daily basis as the release is hidden. You've got to work hard to get to it the first time, but then you can add a pull strap. The seats very easily fold up for a flat floor, but then if you wanted a hidden long storage area you could fold down from the back and store things right next to the back of the cab.