I doubt people buy those specced out models to brag about it online. I think they buy it to make sure it will be able to keep up in 3 years as well. Then of course there are the professionals who buy the specced out models because they actually need the extra juice.
I think the average consumer looking to buy a 27" iMac would be completely fine with the base model. With that said, for those who want to do some gaming and have the iMac last a few years (and still be able to game on it) would benefit from choosing the upper model (and for those extreme gamers or those who want the computer to last a little extra long in terms of GPU performance - upgrade to the 780M).
In terms of storage, the 256GB SSD will suffice for most. Keeping your media external (and backed up) is good practice anyway.
In terms of CPU, most people will definitely do just fine with the i5. Even those who do video/photo editing on their spare time will do just fine with it. Getting the i7 might save you a few minutes on a normal consumer grade video editing -render. Definitely not worth your money IMO.
Memory is user-upgradable.