IF they can be resurfaced, there are no problems in doing so. The problem is that very rarely are the rotors able to be turned, and that rarity has made the shops that do it nearly obsolete.
We've got a lathe with the big arbor capable of turning HD/MD rotors and drums. But like I said, >95% aren't capable due to rust, cracks, or wear. I use the lathe more often to fix brake shake caused by run out or parallelism issues. Most of the time, if the pads are worn, the rotors are too, thanks to the friction formulations that are being used nowadays. Pads nearly outlast the rotors any more.
Brake components always get replaced per axle. Rotors, pads, and even calipers and hoses always get replaced in pairs, as not doing so can and often does create a pull. and then you're redoing the job and replacing both anyways.