Funny enough, I connect a MBP to an iMac and work off the iMac display as main monitor, with wireless keys and trackpad. Overkill? Certainly. But I had the combo, so... Plus, when I'm not leveraging the iMac's display, the kids use it.
A MacBook Air + a moderately priced display is about iMac territory, price-wise. I think there is a legit market for a consumer-priced, Apple branded monitor for those of us who appreciate the flexibility of a portable but find times when a larger display is needed.