I certainly believe that any camera can serve as a backup.... depending on personal uses. More accurately, does one need a BACKUP or a COMPLIMENTARY camera? Two very different things:
My backup to the M9 is no doubt the M8. Its the same batteries, same lenses, same focusing technique, same feel, same operation, almost identical layout, similar advantages, and similar disadvantages. Most importantly, there is almost no mental switching when going between the cameras. Something that was a bit annoying when the Epson R-D1 was a backup to the M8. Obviously the best backup is well.. another M9.

No need to rethink focal lengths because both cameras would be full frame.
Now if slightly switch thought here and think of "complimentary" cameras, the 2x crop factor actually works in my favor. Why? Because I can take the same exact 90mm Leica lens and treat it as a longer telephoto with an equiv FOV of 180mm. See? 1 lens two uses. The M43 cameras also provide telephotos beyond what is feasible with a rangefinder, Autofocus when I want it, AND Macro. All things a rangefinder may not be the best tool for the job.... and with a compact size (body + lens) that doesn't weigh too much more than the M-system by itself.