I will agree that the M was designed to primarily be used as a compact camera that is easy to carry around. And yes, mounting an L lens on it does negate that purpose. However, it is clearly not the only intended purpose from Canon as they would not have released the EF/S to M adapter alongside it otherwise.
To me, the mere fact that the combo of the M with the 22mm is super compact is already worth the price of admission. But to take it a step further, I have the CHOICE to mount whatever lens I want to it. The key here is that it is an available option offered to you from Canon which is more than could be said for most of the other manufacturers with a mirrorless offering at this point.
For all intents and purposes, what Canon has offered is a mirrorless contender (body only on eBay for $240 + Adapter at $115 both new) that gives you access to the super compact as well as the rest of their line of products if you so choose to use them with said body. At $355, it is only slightly more than half the cost of your RX100. Again, at that price point, I could mount whatever the heck I want on there. At least I can say that I have the option.
I get your typical APS-C body IQ (i.e. any of the rebels, xxD, and 7D) for that same price point which is a deal at any size. I personally have used the M as a second body along with my 5D3 for a different FL which would justify mounting an L lens.
Mounting L lenses on the M also gives me distinct differences like a longer macro through the 100L, the 24 tse now becomes a 38mm which I prefer over my 45 tse in some instances. I have mounted my 90mm tse with my 2x III teleconverter to get a 288mm tilt shift lens. Why? Because I can. The possibilities are basically endless. The fact that the M allows me to have these options is great. If you don't think it is for you, then don't do it. But it doesn't mean that it doesn't make sense for plenty of other people to do it.
I could theoretically have gone out and gotten a larger APS-C body so that the ergonomics would have been better. But why would I do that when I picked up the M, adapter, flash, and lens for $500 total (cheaper if you don't want all those things)?
In a nutshell, different strokes for different folks. No need to judge other people on how best to use their gear so long as they are producing captivating images.