It's very rare to see something that fits the textbook definition of a 'scrub' rather than hearing the word used as a random insult, but this is it right here. This is the scrub mentality. Saying that using a specifically-designed game mechanic in the exact way it was intended to be used is 'cheating' because 'it's overpowered' while dismissing their own strategy of choice as 'not cheap' because 'it's what you should be doing.' A bit of a disingenuous argument, that one, because the way I see it if you're in a survival situation you should always be using the most overpowered thing you can because your goal is to survive, not 'play fair.' The creation of arbitrary house rules then acting smug about 'being better' than other players because you stick to that set of artificial restrictions is what defines the scrub.
Do I use junk turrets? Sure, if I feel like it. They're handy for keeping me covered while I do other things, because killing zombies isn't exciting after the first few and I'd rather be doing something else without worrying about turning around and killing zombies. Do I use junk turrets all the time? No, because they have their drawbacks, as FA_Q2 mentioned. They're just another tool in the game, as added by the game designers, and I'll use them or not use them as I feel I want or need to.
Nothing cheating about using the tools provided by the game in the way the designers intended them to be used. It's only when you get into the specifics of AI behavior to make zombies, say, run a loop forever that you're getting into actual exploit territory, since that's clearly not an intended mechanic by the developers.