Like KTM said, most shops do it now, reduces comebacks, and it hardly makes sense to machine a rotor these days given labor costs vs. the cost of a new rotor.
I've reused rotors once and awhile, but only if they're in good shape (no rust, grooving, etc). European OE rotors tend to hold up, vs. other OEs where I find they're typically in pretty bad shape by the time you're replacing the pads.
Also, in the old days, most pads were semi-metallic so they'd chew through the thickness of the rotor, but also kept the surface of the rotor pretty clean...so if you had thickness left, you could run it. Now days, with most cars using Ceramic pads, they're 'easier' on the rotor in that they don't wear as much, but they're also less effective at removing rust and other contaminants from the braking surface. As well, most pads operate with a transfer layer of pad material on the rotor, if that starts to build up unevenly for whatever reason (poor break-in, contamination, etC), you'll get shudder/vibration from the rotor - this happens way more frequently with modern ceramic pads than it did with old semi-metallic stuff.
