They idle for operator comfort. Not to keep the machine warm for starting
I'd imagine if they are idling for operator comfort, then its probably not an all night long, sort of thing, eh? In my experience thus far, the parked regenerations simply can't do a good enough job of keeping the DPF's cleaned out, when they idle for the cold night time hours. But if its just for a slow period during a normal work day, I'd imagine you wouldn't have any issues, as long as the operators keep an eye on the soot levels.
A pro heat is a diesel fired coolant heater. Its a small package that uses a very small amount of the diesel fuel in the tank to heat up the coolant, which is then circulated via an electric pump in the pro heat unit. They have timers, so you can set a timer for a few hours before you're set to start and you'll have a nice warm machine.