I dont understand the rationale for having it be one button.
On a traditional keyboard it's because you might hit the key with either your left or right thumb, and unless you're touch-typing your hand could be in a variety of positions when you hit it. It's the most commonly used key when typing, so making is easily accessible is important.
On the Pandora it's good because it enables you to see quickly where the space bar is, the larger button is easier to find by touch alone, and because it's called the space bar, so it should damn well be a bar shape!