Single cannons actually do have their own hardpoint. That red nipple on the very tip of your saucer? That's your cannon. Turrets are generally a bit further up the saucer, between the centre and edge.

Regardless, I also agree that each ship, at least the larger ones, should have multiple turret hardpoints scattered about the hull. Not necessarily using the beam arrays, but a series of hardpoints that can make a broadside look properly epic.