Turning or not, the rotors have to be flat and true. If they aren't? Braking performance will be reduced.
As for "disposable" rotors and drums, true, many today are cheap enough to simply replace but have you priced out some for performance cars? I seem to recall someone mentioning rotors for a late model corvette are upwards of $1200.00. If the rotor can be re-used by machining them flat, then why not?
As for how they're constructed, true, they used to have the bearing race machined into the rotor and now the rotors simply bolt on. But they aren't cheap and can easily be machined if there is any usuable life left in them. I know my F150 rotors were something like $300.00 a piece. I asked if they could be machined instead of replaced and the mechanic said there wasn't much life left on them (120,000 kms). Seems the new(er) ones are designed to wear along with the pads or some such thing.
As for drums: again, if the face isn't 100% flat and round, reduced brake performance is a result.
But hey, don't take my word for it, y'all go out to your nearest napa, grab yerself a big hammer and have at it....
Just don't come crying back here when you rear end someone on the 401 because you fucked up your brakes.....
Funny: P smerf says "go ahead, give it a shot" in this thread, when it involves stopping a 2 ton vehicle but says "call a pro" when it comes to electrical.......BOTh can kill you if done incorrectly.