Although the idea of connecting a screen and keyboard to a smartphone to work on is a neat concept, it’s not practical yet and I’m not sure it will ever be worthwhile compared to simply getting a separate device. At the moment, you can get an On-Lap 13" screen that you can connect to a smartphone with MHL cables - but it’s a clunky set-up, and at $199 you’re already paying as much as the Nexus 7.
If external screens can combine high-quality, low price and easy connectivity, it might catch on, but at the moment the technology is too clunky and the saving in price not enough to make it worthwhile.