Well, you're right in some ways.
As u said, Labrat, Hell, and Damn has layer 8 comfortable only with 320x240...
But in same way I dont think 320x240 is good enough...
When playing small resolution, some players cant notice hurt stuff, enemies, etc. on his way..
640x480 resolution good in most... Better vision, other background stuff may looks nice(i mean 7,6,5 layers)...
So, playing 320x240 or 640x480 is not a rule...
Its player's decision.