I have a 15" Macbook Pro, so Air / iPad Pro will be in addition to MBP. The iPad Pro appeals to me on 4 (or 3.5) points:

1) As a reader I don't read that much to justify a separate ebook device. I am cautiously optimistic about iPad Pro for documents / articles / papers etc. Also highlighting with pencil.

2) Split Keyboard. Air will force me to use regular keyboard, and I will get RSI after long typing sessions. I think iPad Pro + Dock + small ergo keyboard will be about the same weight as Air.

3) Sketching, drawing, highlighting (via Pencil). Seems nice to have the alternative way of input/draw things / diagram stuff. Also promising for PDF annotation etc.

4) Form factor, seems like it's easier to take iPad Pro to the park to read/ browse / check emails. Not entirely sure about this point though.