Or to put it another way -
16 bit gives you possible values from 0 to 65535 (2 channels each, pan/tilt)
8 bit gives you values from 0 to 255 (1 channel each, pan/tilt)
The two big issues have always been smoothness of motion (no "jumpiness") and precise aiming.
8 bit on Martin SCX gives me .7 degrees resolution on pan position, and .28 degrees on tilt. Good enough for pointing in many applications, - all of mine.
Do the math on 16 bit, and the resolution can be way more than twice as precise.
The other issue - Smooth movements - are handled on the cheaper units by letting the instrument control pan/tilt speed, or not.
The Martin SCX scanners, for example, have an extended DMX mode that lets you control pan and tilt speed to prevent jumpiness. (or not). So even if I send it poorly written, jumpy movement cues, the instrument "fixes it."
The higher-end of the Martin line all has 2 channels for pan and 2 channels for tilt.