MB Air are too big and heavy these days. Go for a Samsung Series 9 or Chromebook, especially if you don't care about Retina. Certainly not the MBPro, they're incredibly heavy and overpriced.
I do basically what you're proposing here, with a lightweight ultrabook, and it works very well.
Is that supposed to be a joke or are you really being that ridiculous?
MBA is too big and heavy? Right...
The Samsung Series 9 is typical Samsung poorly made junk and a Chromebook is a whole different story.
The MBP is not heavy or overpriced for what you get. Show me something else in the PC World that compares..oh thats right, there isn't anything.
I do basically what you're proposing here, with a lightweight ultrabook, and it works very well.