Thanks, I agree with the respective benefits and drawbacks.
A global modifier would be a little awkward to add to PowerDevil, because it would affect just the KWinBrightness displays and not any other ones, but there isn't really a notion of "set a display property for all displays of a given detector". We could still introduce this of course, I don't see a fundamental reason why this wouldn't work.
The other thing with a global modifier is that it wouldn't allow for use case no. 4 in #38, "Dim other screens when game/video/etc. goes fullscreen". Perhaps we don't care enough about this use case to steer the protocol into one direction or another. I did think it's nice to have the option to add it later perhaps (with PowerDevil keeping track of how different dimming multipliers interact with each other per display).
In the end, I think you and Vlad are the people who need to make the call as to what is or isn't acceptable in these interfaces. We'll adjust PowerDevil to work with that decision.