For the vast bulk of stuff that most people will want to do on a computer, though, most of the time you don’t need a desktop monster. I don’t think you even need a 15” screen, which is essentially a portable desktop. You just have to use a small screen productively.
I’ve never used an 11″ Air for work, or anything, but I did have a 13″ Air for quite some time and currently use a 15″ MacBook Pro. What Matt is saying is true for me 95% of the time. I got rid of my 24″ monster screen because I simply didn’t need all of that.
There is, though, one specific instance where I am still cramped working on a 15″ display: working on a large spreadsheet while flipping back to reference information.
Just today I was inputting data into a property tax analysis spreadsheet from Assessor websites and it was a pain. Would have been faster for those two tools to be side by side, but that creates its own problems on small screens.
But, then, ask yourself how much you do that work — likely it’s less than you think. For just about everything else I have found the smaller screens to be better.