That would be overpowered. Everyone would be engineer and destroy friendly MG nests(if that's possible) to construct their own MGs and accumulate points for their teammates killst BUT as far as we know, if a friendly Operative hacks your turret and gets kills, then you get XP. Although this wasn't confirmed it doesn't seem out of the question.
Developers have said on the Splash forums that once a turret is up, it stays up. There are two kinds; mounted guns protect your body but not your characters head, and another is like a standalone turret with a face shield, which protects the head but not the body.