I have some offline maps for which there is no corresponding map server. Also, I sometimes want to load an offline map on my device and I want only the original map--that is, I don't want any tils to be loaded dynamically as I walk around or use the device.
In those cases I just create a completely fake server URL. If I am in an area and at a zoom level where I have a tile in my RMaps file, everything works fine, and if I move out of map coverage or to a zoom level I don't have, I get an invalid mapserver URL error... Which is what I want.
My point being, that the mapserver URL field has to have something in it, but it doesn't have to be a valid URL if you don't want it to be.