I tried having the 17 inch MBP as my only Mac and prefer the iMac and MBA combo. But there is something about having a single Mac for "everything" that is appealing.
Edit: I currently have the iMac 27 i5 and a Ultimate MBA 13. Sold the 17 inch MBP.
If I was going to have a single Mac I think I'd look at the 17" too. Never used a 17" for long but the thought of having a large mobile screen is appealing at times.
When I used a 15" MBP although the screen size was nice it took away from the mobile experience compared to how great the Air is for 'everywhere' use.
The Air on the other hand now has the same screen real estate as the (standard-res) 15" MBP and it's so small and light one never thinks twice of bringing it, grabbing for it, using it anywhere and enjoying it.
The 17" doesn't offer that great 'mobile experience'. Its is heavier and more cumbersome to pull from a bag, not the most comfortable on your lap compared to the Air, takes more space on a table ... it doesn't scream take me everywhere like the Air does.
Since the Air is so portable and fun to have by your side it's saved me a lot of time and increased my productivity. If I'm checking email from my phone or ipad and I see some emails need a long reply I'll grab for the Air and bang off a reply real quick instead of slow touchscreen typing ... even if I'm on the road or traveling long distance the Air is readily and easily available to grab and start working. If I used a 17 or 15" I'd probably plink away on my ipad keyboard instead ... same for around house use, taking to bed whatever.
The Air is the sweet spot, MUCH lighter than a 13, 15 or 17" MBP adequately powerful to do real work, and it's just as easy to grab as an iPad. The difference being when you're on the Air you can fly through ALL your work, the iPad limits you ...
Having a desktop back at the office for heavy lifting, nas, backup, storage, a 'real' large 24/27" screen completes the setup perfectly. Whereas if you're relying on the 17" MBP only and you're mobile with it then you've lost your home nas, storage, back up machine because your desktop is disconnected and in your bag, any downloads you where doing etc have to be paused ... it's just not a convenient ideal set up.
With Cloud Storage like Drop Box, Mobile etc out there now it's too easy, it's all automatic sync and for me it's been flawless. Dropbox allows 100gb of storage, (yes for a fee), ... access to 100gb is all I need when I'm mobile .. a little usb HD could be used for more data.
The MacBook Air fits my needs REALLY well, it's a heck of versatile notebook with all kinds of power. Add a desktop to that and you've got the best possible set up going, imo.