The 'light sensor' is in fact the backlight LEDs, as LEDs can be used to generate small amounts of power when exposed to light. #TMYK
a flat (pancake) motor is going to move the weight side to side and front to back—which would slide the watch along your skin, and make it push/pull against the bands and buttons, creating noise (especially on the metal bands).
a more traditional cylindrical motor, with the orientation they're using, is going to throw the weight up and down against your arm, increasing the tactile feel of the vibration. They could cut the amount of vibration quite a bit to reduce noise and still have a stronger-feeling vibrator, and the orientation they've placed it in means the buttons will most likely rattle less.
I've never messed around with a pebble steel, but I know they tended to get loose and rattle on the regular pebble.