In PC world you would be right.
Three days ago I exchanged my iMac 2009 (!) 2.8 Quad Intel 860 8 GB ATI 4850 128 GB SSD for a 2012 i7 3.4 16 GB 16 GB GTX 680 MX 250 GB SSD.
The first point I want to make is: I got a good deal, meaning, iMac keeps its trade in value. A PC wouldn't - after three years it's worthless even if you put the highest grade components in it.
The Mac (and iMacs) may not have the latest of the latest but in combination with the software it works. To be honest with you, in day to day work I hardly notice any difference between my old and new iMac. Yes - Handbrake is much faster and so are other CPU and GPU intensive tasks but in my case, iLife, movie downloading and unpacking (and sometimes decoding), Parallels Windows for work etc even the old iMac was sufficient. I am still looking for an app that can unrar for me and give me much higher speeds (now 20% gain)....
Yes, there are newer, higher grade components. Yes they are faster than previous gen. Even the new vs the old iMac gives a higher score in geek bench. But then again - do you see the actual difference?
My benefits now are Thunderbolt, USB 3 etc and on top of that I am now able to install Windows 7 64 bit in Bootcamp and score a 7,9 system score out of 7,9 maximum, hence being able to actually play games in high res. That my old iMac couldn't but it wasn't important for me at the time and even now it is an " extra".
The 2013 iMac will of course be even better, faster etc etc. So will the 2014 and 2015 by the way.
I have - since I made the change in 2007 - been happy with everything iMac brought me in terms of performance, usability and functionality.
But then again - depending on your needs - there is also something extra to wish / hope / need for.