This is different from having the turrets completed only in that the turrets will have less time to get shots in against the AI ships, I'd say. I think this would not work to your advantage, because as soon as the turrets are up the AI will retarget to them if they were going to (I think...). Plus, turrets are extremely low on the priority list already, so more likely the AI would kill the important thing and then fly off to wherever.
I did notice the AI seems to target nearer, lower level, still-building turrets over further away, higher level, actually-attacking-them turrets for some reason. It looked like the further away turrets were in range, it just didn't seem to want to attack them.
It does prefer to kill off stuff that it can kill more quickly over stuff that is still at full health, so that's probably part of it. That's a good point, though, it probably should not do that.
I'm guessing it's a holdover from the old economic system, where a quick plasma bomb could wipe several hundred resources off the balance sheet of a careless/over-aggressive turret-builder? It's a lot less costly to splat down a bunch of turrets around enemies stuck in a tractor beam and hope some finish building than it used to be. Might it make sense to put in a "can't build under fire" delay of some sort, similar to the "can't repair under fire" (but perhaps even longer?)
I've been thinking about the low->full power warm-up period that I saw proposed earlier, and it's making more and more sense to me, at least for fixed units (and especially shooty fixed units.)