Leica R lenses
When you have a M camera you probably have a bunch of M lenses.
So all the equivalent focales lenght (wide to 135mm) won't be really interesting.
Only the few who have long télé might use this possibility but the M would be clumsy with those big lenses.
Now you may have "R only" owners who, in despair of not having a R10, may go for this combo M body+R lenses, it's a bit odd.