Think about it... The more mobile we become everyday, would you consider using only an iPad Pro given the size and leaving the Mac in a drawer... Completely switching to tablet base?
A 13" iPad could have a desktop stand and when you want to... set it on the stand for desktop use... with a keyboard and mouse - both bluetooth... and you'd have the best of both worlds. And just think what it could be if it were a phone too...
Don't have a Mac, yet, still with PC and my iPad now already replaces a lot of what I would do on the PC. Even though I think 13 inch would be harder for portability, I think it could come in a close second to a Mac.
I use a bluetooth keyboard on my iPad and to be honest, it's just not the same as having the functionality of a laptop with me. I'd rather splash out on a new Macbook, preferably the rumoured Retina Macbook Air i've heard whisperings about on the intermawebz, or the current Macbook Pro Retina if the aforementioned Retina Air never comes to pass.