when the filter gets scratched up, then replace it with the MRC.
I use filters because I DON'T use a lens cap much if I'm out and about... with the rangefinders I end up with a few blank shots where I forget the cap if I do use one, so I usually just put the lens on the camera with a hood and filter, only cap it when I'm not using it.
The MRC's take a LOT longer to scratch up then the regular ones in my experience.
and yes, if you're going to use a UV, use a nice one.
(ps- I'm much less likely to use them for my SLR gear, simply because I always know when my cap is on!)