As I wrote earlier, my MacBook Pro and my 12.9 iPad Pro serve different functions. My MacBook Pro is for editing complex documents and such, my iPad Pro is for using as a virtual whiteboard when lecturing to my students, using as a digital notebook for mathematical musings, and anything that requires the pencil.
It has occurred to me it would be nice to have a device that could do both of these at once. It would be nice to have a MacBook that I could fold backwards and start writing on with a pencil. It would give me one less device to carry around, and it would solve some issues I have getting data back and forth between my MacBook and iPad when WiFi was flaky.
And of course, some of the windows two in one devices do exactly this, particularly the surface book. But when I've researched these devices, it has seemed to me they are neither as good at being a laptop as my MacBook Pro, nor as good a writing-tablet experience as my iPad Pro. And I'd rather have two separate devices that do these tasks to perfection rather than one device that does these functions ok. In theory, a two in one device would be the best solution. Perhaps some day someone will create a two in one solution that does both tasks so well, I'll switch to it. But given I'm choosing among solutions that actually exist, the MacBook/iPad Pro combination works best for me now.