a variable capacitor will present some mechanical resistance and thus conversion of mechanical energy into heat, at low speed this is negligible (unless you need the system to behave reversibly), but if you are going to rotate your variable capacitor at speeds where frictional heating becomes an issue (perhaps the generation of heat limits your application, or the loss of mechanical energy limits your application) then yes these types of dielectrics will be of interest to you.
Otherwise, conventional dielectric can be more compact for the same maximum energy storage in the capacitor, since no simultaneous optimization on viscosity was performed in their selection, resulting in a wider range of dielectrics being viable.
How fast do you plan to turn your variable capacitors?
I wasn't interested in the rotation speed; more the high voltage - I know there are vacuum variable caps for high voltage work, but don't know what the current state of other dielectrics at a few kV is.
Otherwise, conventional dielectric can be more compact for the same maximum energy storage in the capacitor, since no simultaneous optimization on viscosity was performed in their selection, resulting in a wider range of dielectrics being viable.
How fast do you plan to turn your variable capacitors?