The biggest and main advantage of the portable models is just that - portability.
and the ability to mount it in it's protective shell when on the move. I wouldn't want to keep moving my mains-powered external drives around from place to place - too much hassle.
I have a 3 year old seagate 2tb usb3 drive that I've had no problems with. This is the book sized external with a 3.5" drive inside that uses an interchangeable dock that you can also get in usb2 or esata forms.