I would love the second monitor option too.
I've just purchase and I love Astropad, including now with the latest update who made possible for me to use it over wi-fi (yeah, I couldn't till yesterday). But I would love to see some things on my macbook, like references, iTunes, videos or whatever, without having to swipe between programs every time.
Right now, I do that using my iPhone on my workflow too, and it is not cool. Specially if you think you have a computer right on the side that serves you "just for the keyboards".