I can find only one reason for Apple making the PowerBook thicker (like, .2 inches, or so), and that's if they added a hot-swappable drive bay. That is what most professionals loved with their older PowerBook G3's, and made them reluctant to buy a PowerBook G4, even though you could attach almost anything with firewire.
A hot swapable drive bay could allow for 2 batteries (redesign the batteries), and future implementation of a superdrive.
I'm skeptical about the superdrive, though, because if I remember correctly, when it first debuted, the drive itself was bigger (longer) than other CD drives. I do also believe that it would use way too much power, and that it would heat up a lot.
These are just my opinions, but I don't think that many people would mind a little thicker TiBook for a hot-swappable drive bay...