I used to find the different subsystems with different dice illogical and irritating. Now I think I detect an underlying logic and this seems to be the place to ask: is it something like this?
Multiple Dice (e.g. 3D6): Avoided except for chargen and dramatic damage e.g from falling or big monsters. Lots of dice being added slows play, and makes it hard to manage multiple characters. Stats act as quasi hit points for Stat Bonuses, e.g. depleting endurance can deplete Hit Points.
D6: The default. Easiest dice to read and handle, particularly economical when there are lots of rolls to be made, e.g. party blundering through dungeon looking for traps.
D20: The second default, for when Stat and Level bonuses can effect the score, e.g. saving throws and combat.
Dx (e.g. D4): Used instead of modifiers. Raises or lowers the ceiling, but maintains a floor of 1.
D100: For when the focus is on the character because of having special abilities (e.g. Thief.)
Behind this seem to be the design principles:
- For routine rolls, use a single dice, defaulting to D6 but using other dice in preference to +/- modifiers. This speeds up play, and enables the DM to handle multiple characters or repeated actions.
- Where the focus shifts to a particular character, use % or multiple dice. This creates more numerical nuance, but also the increased physical/mechanical difficulty enforces that shift of focus.