Turrets should be tied to kits
I think this is a bad idea.
Turrets aren’t useless – just not up-to-par with some of the other things. I, personally, have used all six of the turrets at one time or another, and will continue to do so, especially since it seems like they’re beginning to be allowed to scale properly. With scaling, lowered cooldown, and perhaps rebalancing of the toolbelt skills, Turrets could easily become something far more interesting than they are now.
Tying turrets to kits would bring up another question, besides – how would that work? Would each Kit’s toolbelt skill be replaced with the Turret? Would one of the attacks on said Kit be replaced with the Turret? How would Turret detonation work?
Not everything about the Engineer needs to be tied to Kits, besides. Some people prefer to not use Kits, for one reason or another.
Better to bundle them into a Turret kit (with a single toolbelt skill) and free up the utility slots.