So, do you spend double the money on a lens you can only use on your M, or do you spend half the money on a lens that will also work on your rebel or your 7D?
I think if you fit anything other than a pancake to an M and you've lost the size benefit.
I don't have any APS-C cameras other than the M, so compatibility with them isn't a benefit.
The M + M11-22 or M18-55 fits in a small pouch on my belt. The SL1 + 10-18 or 18-55 would not.
Guess I take a broader view of compact system cameras (CSC). It's not just a pocket camera to me - its a compact "system".
Exactly. Consider my full 'M kit' – EOS M, M22, M18-55, M11-22, 270EX II – compared to the equivalent dSLR kit, e.g. SL1, 24/2.8, 18-55, 10-18, same flash. The latter is much bulkier, the former fits nicely in the compact ThinkTank Mirrorless Mover 20.
My complete M kit takes up about the same amount of space as just my 1D X + 24-70.
I just returned from a trip to Europe with my family (wife + three young kids). The M + 11-22 on my belt and the other two lenses in a regular backpack (along with snacks, diapers, etc.) made it very easy to get around with the family during the day in Paris, and late at night I went out without kids but with tripod, 1D X, 24-70/2.8 II, TS-E 17 and Rokinon 14/2.8.
Lots of size benefit to the M ecosystem...