Leica RF were made for fast, decisive shooting. Having a separate hand held meter makes the use of the camera awkward. Tough to hold both a handheld meter and camera and then take a reading, and then transfer it to the camera. With the MR-4 meter you only need to make one adjustment, either move the shutter speed dial OR the aperture to center the dial.
The MR-4 meter circuit will compensate for a the decay curve of a silver oxide battery. Generally all you need to do is adjust the screw on the bottom to zero the meter. I've had a number of the meters over the years and this generally works though sometimes you can't zero the meter. In that case you need to use an adapter.
I use hand held meters when I shoot 4x5, not 35mm. The only downside for me with the use of the MR-4 is using wide angle lenses that require a separate VF.