An MBA and an iMac are more expensive than just an MBP, but since there are various models of MBAs, iMacs and MBPs, the prices differ.
Have you looked at the price information Apple offers on their Apple Online Store for your country and tried doing the math yourself with the exact Macs you have in mind, as there are various models of MBAs, iMacs and MBPs, thus the prices differ?
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?