Personally I wouldn't buy a laptop if I knew I was going to use it mostly as a desktop machine. I'd buy an iMac and maybe a 16gb iPad if you wanted some portability.
However that doesnt make anyone else wanting to use a MBA as a desktop replacement wrong or misguided. It is down to personal preference.
Definitely personal preference, but through my own experience, I'm much more wary of recommending this to people.
I had a 2006 17" iMac that fit my needs just fine, but I wanted a mobile solution. Since I'm a casual computer user, the iPad was the solution. The first few months was mostly internet, email, and games, but I found myself using Pages and Numbers on the iPad more and more.
It got really frustrating not having all the features of Pages/Numbers for Mac (I knew the limits going in, I just didn't think I'd want to use them on the iPad very much.) Also was a pain to remember where the most recent version of a file was. Did I make a change on the iPad? Yes; plug it in, transfer it to iMac, make edits with tools the iPad version doesn't have, save. Transfer back to iPad so most recent version is on both devices. Pain.
I sold the iMac, sold the iPad, and have a 13" MBA on the way. No more managing files, no more wishing I had all the features Pages/Numbers offers. But still very portable.
The MBA makes sense for a lot of people, and I'd wager, it makes more sense than a desktop + iPad for a lot of people.