They have no downside, but their upside isn't very exciting.
I want to love core turrets, I really do. They were one of the features I was most excited about. But I can't get behind the idea of having to choose between what few core turrets I have, and every other turret I have. I am the kind of person who drops a few of each kind of turret on every planet because each turret is useful for different things; I'm not going to give that up for a pile of one type of turret. Even if they are stronger, it's less fun and interesting. I don't want to go from "Okay, I'm gonna drop my missile turrets back here to cover cmd and the wormholes, then we'll build a nice little tractor/ff/flak trap on the wormhole, and here's some more turrets HERE in case they go after THAT..." to "Meh, giant pile of MkV needle turrets. Defenses done."
I also think if you want to encourage distributed defense, the way to do that is to make all turrets distributed defense. Trying to shoehorn MkV turrets into "absolutely must be distributed defense, and absolutely must not be used to chokepoint" feels very... jury-rigged.
EDIT: Clarification to the above: I am NOT suggesting removing chokepoints, or even really nerfing them. I'm just more and more thinking it would be awesome if turrets weren't the sole thing that defined a chokepoint. I really, really like Orzelek's idea, basically.