1. Balance, what most people here are complaining about.
Again, A turret AND a human on it even with rifle is more effective than a turret itself.
2. How feasible is it to code?
Its not that hard. As far as I was doing modding this will require a simple view position change and a angular speed limit. Second can be a thing to think about though. Still not hard.
Simply, we have an already programmed defcomp, turrets, and humans, what we need is to take human view into the turret and assign his controls to turret... (at this point /me went to check out the tremulous SDK)
3. How intuitive would the interface be? The 5 second login-logout would actually encourage people to hop on a turret, and spam with it until it dies, instead of waiting 10 seconds just to pick a new one.
Press Q on defcomp.
Scroll using scroll wheel.
Press Altfire to enter/leave a turret.
Why loginlogout? Because then you will have a turret hoppers fenomenon

. And on contrary, the spread will make it easier to kill a dretch, because you cannot aim fast! Your crosshair will be moving slowly and its not even like a low sens where you can swing your mouse over the other side of the table.