Currently momentum sources can be only defined as flow speed vector on XYZ axes. As they can be used as simplification of MRF zones, it would be useful to pace them similarly (as MRF zones) with CSV file.
Definitely more dynamic simulations could be run with that option activated. Implementation of multiple MRF zones is time consuming (already while creating a geometry) and simulation core-hours hungry: it could be avoided with switchable / modulable Momentum sources.