Big Pistons Forever:
There is no reason a an RMI can not be used for tracking a VOR radial. It is usually not required because you can just follow the track bar, but on one aircraft I fly the Capt has Nav 1 on his HSI and Nav 2 on the RMI and the FO has Nav 2 on his HSI and Nav 1 on the RMI.
I am well aware it can be done but there is a good reason not to be doing it, at least when circumstances are critical, such as on a VOR instrument approach's final approach segment. The FTE assumptions used to provide protected airspace for a VOR IAP's final approach segment does not make allowances for the additional FTE that results from using an RMI instead of the CDI.
Because of the simplistic manner in which VOR IAPs are constructed, there is a geometric pad unless the final approach segment is at the maximum permitted distance from the VOR facility.
All the air carrier aircraft I flew had dual pointer RMIs for both pilot positions. We used them only for reference with VOR, but they were primary for NDB IAPs. NDB criteria make allowances for the use of an RMI and, for that matter, a fixed ADF compass card.