Fat lot of good when your laptop ONLY takes SATA (M2 pretty much has to be built into laptops). And desktops will have a hard time using an add on when the only slot that can carry it runs the GPU.
Then you aren't buying the right laptops or desktops.
While that's the case now, I would expect things to change in the future as trends change, as always happens.
One of the motherboards for the new Threadripper CPU (admittedly not exactly in the standard consumer class) comes with 6 M.2 x4 connectors, 2 onboard and the other 4 from an included PCIe add-in card.
Expect this sort of thing to creep down into the consumer space over the next few years. Some consumer motherboards right now have 2 M.2 slots onboard. If M.2 becomes the consumer standard such that it displaces 2.5"/3.5" form factors, then expect motherboards to ship with more.
Yes, if you want 4 M.2 right now on a consumer mITX board that only has one long PCIe slot for a GPU you are SoL. But if you want that many M.2, then you need to purchase a motherboard that can support that many, just like if you want 10 SATA drives you need to purchase a motherboard that can support 10 SATA or has additional PCIe expansion slots to add more SATA ports from add-in cards (or purchase one that supports SATA expanders, which is pretty rare in the consumer space).