I tried to use it (for fun) at some point but the API design felt too verbose/cumbersome for what it did and by the time DrawPrimitive was added in D3D5 it was

I tried to use it (for fun) at some point but the API design felt too verbose/cumbersome for what it did and by the time DrawPrimitive was added in D3D5 it was simpler to use that directly than use D3DRM. It was also very clear that D3DRM was designed by a different team than D3DIM.