A very interesting thread, and I must say I am really impressed with the thoughtful and helpful tone in the responses to the OP.
To the OP: I have a MBA and (recently added) an iPad mini; I use both, but I do different things with both. Essentially, it is the difference between content creation and content consumption.
The iPad is a wonderfully portable device for surfing and browsing - however, to me, it is not a tool for serious work; for that, I have the MBA, and, as my job requires a lot of research and writing, I need a proper computer for that. As I travel a lot, that computer needs to be extremely portable and powerful. Hence, a high specification portable MBA.
However, as you have said you actually have an iMac, (which you use) you may not need another computer, even a portable one, in which case, it may make sense to keep the iPad.
If you keep the iPad, I recommend that you get 1) a good quality case for it, and 2) add a small, portable keyboard to it. Personally, I'm not crazy on touchscreen typing, and much prefer to use a proper keyboard.