a grenade and cover make extremely short work of any turret.
You know, I was thinking about this while I was waking up this evening, and I realised something. I've been thinking about turrets that way too - but expecting they were intended to be like that, and knowing that if they're placed well enough to catch someone off-guard, they can do a pretty good job of tearing someone up.
But when you think about it, even if a turret just sits there for a while before someone uses a frag on it, the Engineer has come out better-than-even with his opponent. Engineer uses one pip, and a few seconds setting up the turret. The enemy uses one pip and a few seconds lining up their throw and waiting for the frag to take down the turret. At that point, it will still be there, just disabled - the Engineer can now repair it for free, forcing the next enemy to waste ANOTHER frag and few seconds. Unless, of course, your first guy decides to waste more time putting it down properly (thus making the Engineer gain more of a delay against his enemy). If they don't want to camp on top of the turret hitting it with their gunbutt/knife, then they also have to use ammo (and if they're a Soldier, this means a choice between slightly diminished ammo or using 2 supply pips to kill something that only took 1 to place)