I woud guess that the rotary train was about the only operation that would regularly need a cook car. Jimmy Blouch posted the following summary of the last rotary operation in 1962. Note that it took two days for the rotary to open the line, and they spent the night at Sublette. Which was probably not unusual when they ran the rotary. So sleeping quarters and a cook car were a predictable necessity. And the rotary train needed to be ready to go on short notice. So having assigned equipment probably made sense.
Clearly when needed for another purpose, the rotary train cars could be borrowed but the fact they were assigned to the rotary train probably helped ensure they were returned to their normal location reasonably promptly. Here is a picture of the rotary train's enginemen's bunk car being used as a caboose (the normal caboose was bad order in Chama).
JBWX