I would expect it's to allow for a necessary amount of overlap of your skin when holding the phone. We have hands, not pincers. Try picking up your phone, now hold it with the screen facing down, like you would on your back. If you have a good grip on it, your fingertips will probably be overlapping the front edges of the phone. Now try holding the phone in that face-down position, but this time only by the outer edges without wrapping your fingertips at all around the front. You will be able to do it, but it will take a little more strength to squeeze the sides of the phone. That extra squeezing will cause your fingers to fatigue much faster than if you had the extra leverage of simply allowing your fingertips to overlap the edge slightly.
When I pick up my phone, I grab it that way. They never do in commercials, probably because they want to to see the entire device. But when picking up an object it's natural to allow your fingers to wrap around it, even slightly.
If the phones were designed to have the screen run all the way to the edge, it would require users to pick up the device with a more unnatural "pincer" type grip, or else risk touching the screen and thereby launching an app or swiping to another screen or just generally launching some unanticipated interaction.
Granted, my example is only when holding the device face-down, but this can apply just the same when using the phone single-handedly in any position. I love my Evo, and I always want a good grip on it. It would irk me if I had to always watch out for the screen edges. As it currently stands, my Evo's home button has sensitivity that runs a little too close to the left edge for my liking, I find that my palm sometimes activates it during one-handed use.
I would expect that a touch-screen device, that entire dispenses of a bezel, would tend to be a pain to its users. They would either have to engage in one of the two aforementioned tactics to hold the device, or they would probably tend to purchase a case with a lip on it that prevents that type of mistake. And come to think of it, if that were the situation where there was no bezel, there could be no sturdy case for the phone, because there would be no place for it to securely wrap around without overlapping the screen. In either situation, I would avoid such a device.
