I use the app "duet" to connect an iPad as a second monitor for the cases when a second display is really useful (like playing spot the difference with a site vs Sketch design).
This would be wonderful I'm sure with one of the newer 12.9" iPad pros.
And for 1/10 of the cost you can pick up something like a "15.6” Widescreen Flat-Panel USB 3.0-Powered Portable LED Monitor - $99"