Another question is what should use Defensive Supply? All offensive turrets obviously, but what about:
Maybe Fortresses should actually supply a small amount of DS instead. So they could support some turrets even if a Mobile Builder wasn't present.
For sanity sake I'm going to call it Defensive Control, as supply has an in-context meaning already, and will still apply for K-Raids and similar activities. I already have enough problems differentiating between Bomber Fleet and Bomber SS and Spire Fleet Ship vs. Spirecraft in my AARs...
Mines should be control-less. FFs and Counter turrets too, in my mind, for sanity's sake. However, anything that 'defends' the system should be given a control value. I like the idea of Fortresses (and perhaps MRS) being given additional control values, but I could break the mechanic like that by overwhelming a whipping boy with fortresses. It could add in a pretty significant variable. Personally I think that simply lacking a control value and needing supply will be enough to satisfy them for their price/power in this scenario.
Of course, the 'support turrets' to me for a fortress is ~20 snipers for anti-polycrystal work, which would be ~40 of any other turret in theory once balanced.