The problem with non-generic instruments is that there is such a huge variety of behavior among airplanes...if Plane-Maker were to have options for every possible plane, the "special equipment" screen would require 3000 tabs and be completely unusable. Hence the need for a smaller unit of modeling: the lego brick.
The prop disc is the first feature I have done that is meant to be used only by a plugin, e.g. "lego brick" code. The X-Plane systems code sometimes suffers from the same "code bloat" problem as the instruments: a ton of very specific, very tweaky behaviors that interact in strange ways and become very difficult to manage. It's not that the systems code is bad code - it's that the scope of the problem is simply too large. That is, you can't expect X-Plane to cleanly simulate the systems of every airplane ever in a ton of detail through an a la carte menu of check-boxes.
The idea of the prop disc is: someone (LR or otherwise) can write a plugin that encodes a certain style of prop disc. That plugin can then be picked up and moved around like a generic instrument between planes (perhaps with a corresponding text file to control it).
If someone else comes up with a different/better prop-disc algorithm, compatibility isn't an issue...that person writes a new prop disc plugin and the airplane author selects the one desired. Think of it as sort of a portable flight model that stays with your plane.
So we win in three ways:
- Anyone can write the prop disc algorithm, not just LR.
- The code lives with the plane, to avoid compatibility problems.
- More than one plugin can exist, giving authors an a la carte menu.