To start off, I think it's important to realize that the arguments typically only hold valid for refurb/used/heavily discounted prices... which aren't hard to find per se, but that's the trade off. And also important is that if you go MBA+iMac, you're paying for TWO computers, so obviously it's going to be pricier than just an MBP.
$829 for a refurb 2010 11" base MBA
$1019 for a refurb 2011 21.5" base iMac
$1439 for a refurb 2011 27" base iMac
So you can get an 11" base MBA and 21.5" base iMac for under $2000 or an 11" MBA and 27" iMac for under $2500
Now I think typically the argument for the combo is against a 17in MBP, because you're breaking middle ground (11"+21.5" = 32.5"/2 = 16.25" or 11"+27" = 38"/2 = 19"). Refurb 2011 17in MBPs start at $2119 and go all the way up to $2849.
So I think by tossing these numbers around, you can see where people would rather go MBA+iMac than just a 17in MBP.
Personally, I'm just using a 2011 27" iMac and an iPad 2... but I think I may go for an 11" MBA when they get refreshed, as I miss the whole computing experience while lounging around my home, ya' know?