What if portability is worthless? The main purpose of my home Mac is as a media server for the several AppleTVs in the household. If I want to surf the web portably I have an iPad for that. I need a computer that's cheap for the power and storage it offers, that never moves, that never turns off or goes to sleep, and that features a comfortably large display for when it's in use to edit video. The iMac meets every one of these metrics, while any Mac notebook fails all of them until you add a $1k external display, and then it fails all of the metrics except that one.
Not everyone has the same computing needs as you.
I do have a MBP, but I bought it to tide me over between iMac releases because my older iMac wasn't making it to September (or later, or 2013) as a primary household server under that task load. I'll flip the 2012 MBP when it's time, and lose relatively little money because it's still current and likely still will be. But make no mistake, its portability is availing me little, and I wouldn't pay a single dollar for that aspect on a permanent basis.