With nearly 30 years in the business, I can confidently say most guys don't know how to turn rotors. That's where the myth that turning rotors let's them warp came from.

They turn rotors wobbly, you won't notice it at first but eventually the pads try to wear the rotors straight again and the crookedness shows up.
As to Gus' pictures , here in the Salt belt, rotors tend to rust away before getting wrapped. By the time you turn out all the rust pits, the rotors are under spec.

I have my own lathe, I turn my own rotors and drums.