I use both displays. Sometimes I use primarily the MBA with an external display turned on, writing directly on the laptop. However, sometimes I prefer to use an external keyboard and mouse — in those cases I'm editing a document on the external monitor, and watching the content I need for that process on the laptop. A few times, I use the clamshell mode (when I'm just browsing or seeing my email, and I prefer the bigger display of the external monitor). It is wonderful to have so many solutions nowadays.
Now let me be provocative: I can't understand how people don't get tired of working on iPads. It fails all the ergonomic tests (at least for me), and having to touch the screen and type is horrible. Tablets are cool for media consumption, and to do some creative tasks. Ipad Pro's are great to accomplish that with even more detail and quality. But for desk work, it is a total ergonomic failure, in my view. I've tested cases, stands, pencils, and I can say one thing: working on a computer is way more relaxing. Tablet owners, Pilates yourself!