not comparing likewise products.
surface book is similar to rMBP in size and power and what it appears to be up against.
rMBP + iPad Pro will cost a lot more.
the Surface Pro line is closer in performance and price to the MBA, so it would be MBA+ iPad pro for that comparison.
the Surface 3 is meant to compare to the iPad Air.
However, you are right about user preference and workflow. some people might prefer having 2 devices, some will prefer the convergence of one.
consumer choice is a great thing!
I only use my Surface 3 as a laptop after 5 months of trial (some proprietary softwares my company is using require Windows). It is too heavy to be used as a tablet and Windows has very poor battery management. The battery doesn't last long and the device gets warm/hot easily.