I am thinking the greatest advantage of the 5DIII is the ease of usage in challenging condition. Sometimes you miss some shots because of the limitation of the camera, e.g, slower FPS of the older 5D series, and missed or slow AF just to name a couple. These may means alot to some people. There is no right or wrong answers to owning newer cameras. Lastly, in order to produce the picture, you must be able to snap the picture in the first place. If you are not able to snap, even the most expensive camera is useless. I think the 5DIII excels in these areas, not so much in pictures. Imagine in a low light situation, flash not allowed, and there's a moment in that location that warrant any pics taken there and then to be priceless. I certainly won't wanna miss that because my camera can't AF in time.
I will upgrade because I face these issues at times with my classic. But I still love my classic to bits though