They're only incompatible with lower-mark versions of that type of turret. For example, Core Sniper Turrets can't be on a world with ordinary Sniper Turrets. But a controller allows you to put an entire cap of mark V turrets on every single one of your worlds, energy permitting, which is godly for Snipers. In my last two games the AI has had Munitions Boosters, so I've leeched enough to put a Mark II on every one of my planets in a cluster with my Core Snipers, buffing them to 200% damage. Essentially, I've got 10 mark-cap's worth of sniper turret firepower on every planet. You can also build the turrets on any planet on which you've got supply, potentially softening up CPAs by beachheading adjacent planets.
Wow, that's freaking amazing! OK, I'm sold...
The unit cap on Core Turrets is half the normal turret cap, meaning you get 2.5 mark-I-cap-equivalents out of a cap of Core Turrets. So Mk I + Mk II is slightly better than Core, and Mk I-III is more than twice as good.
But yeah, Core Turrets can go on every planet, so you can actually have both a whipping boy AND enough defenses on other systems to reduce your worries about stray ships, threatfleet, etc.
Indeed, hacked one while playing last night and I have to say there's a lot to be said for having "free" distributed automated defense on every frontier world. It almost feels overpowered, although I suppose the lack of ability to meaningfully concentrate it effectively makes it less so. It has very little impact on wave defense, obviously, but it frees up fleet resources from picket duty in a significant way everywhere else, which is pretty awesome. I can understand why these would be a priority, I'll be making them one in my future games as well.
