On tablets it doesn't much matter: the light output of the screen overwhelms any bezel contrast effects. Especially on the larger models.
On eink, though, the low screen contrast compared to the bezel will be more noticeable.
They really are different creatures.
As to the bezel contrast effects, they are real but taste dependent.
Some people wrap light ropes around their TV screens to minimize it or go to hacker route to copy the old Philips Ambilight tech, while most people don't care or don't watch TV in dark rooms.
http://en.m.wikipedia.org/wiki/Ambilight
Me, I prefer flat black on my mobile devices strictly as a matter of taste.