Come off of it. Have you seen the benchmarks that the 680MX is putting out? It is producing outstanding results in very demanding games, especially when you consider that it's an all-in-one and not some full tower with an SLI game card setup.
For $150 the upgraded graphics card is almost a no brainer. It will certainly extend the useable life of the machine if the person has even a passing interest in gaming, video or photo work.
To give a similar example, the machine I have right now is a 2009 i7 iMac with the upgraded 4850 graphics card. At that time the 4850 was only available in the top spec machine. Because I have the 4850 I can still play some of the newer games nearly three years later. If I'd gotten the lower spec video card, then games like Diablo III, etc, wouldn't even be an option as they would perform terribly.
The 680mx is in my opinion a gaming card. The 675mx for 3d grapics professinals would be sufficient. After ordering the 675mx the only thing I am worried about is will 1gb of ram be enought on the card. Bearing in mind I will use the saved money on a new tripod for my camera and a few other items.
I just can't see myself pushing the 675mx to the extent I will the i7 cpu which will be 100% load for hour's on end. If I can push the gpu that hard then I will be saying well done to me.
On a side note the iMac is about minimalistic simplicity, so jamming it full of components we just dont need just defies all logic.