They fit on more heavily defended ships like the Orion. Don't worry, the Centurion is as mean as you can get in Privateer Remake (well..as soon as we balance that Demon). You don't need level 5 shields to lay waste to squadrons of pirates and kilrathi.
You can switch to turret view to command your turrets. Or you can set them to 'weapons free' mode with cntrl+w (turn off the mode with cntrl+r). The most useful key for turrets is Cntrl+t, which tells your turret gunner to fire on your current target - it works much better than weapons free mode. You can select a hostile target, cntrl+t, and then switch to another target and let your turret(s) take care of it.
And, just so you know, turrets have their own reactor. The recharge value and storage capacitor is listed in the description. Having your turrets firing constantly in no way impedes your ability to fire your primary guns from the cockpit.
Enjoy! In the Original Privateer, there was no turret AI (you had to control them by yourself).