What is the point in giving a system 4gb Vram on a GPU designed primarily for gaming (Its not a Quadro) if you cannot use it. Its just marketing BS. Apple know it. Nowhere was it stated that a function available on a 2011 iMac was not available on a 2013 model. You can blame Nvidia all you want but Apple made the choice to go with them not ATI knowing this would be removed.
So the feature was removed from Nvidia? If it was Apple that removed the feature i could see your point but if the fastest mobile video card on the market (the 780mx is) does not have that feature how can you blame Apple?
I have the 2012 iMac with the 680mx and i used it to play games for a while but in the end it was much cheaper to just build a dedicated gaming rig, for $1500 Aus i built a rig that has much better graphical performance then my iMac, it also doubles as my HTPC running XBMC. I now use my iMac for my work in the Abobe suite and web browsing ect.