You said it yourself, i5 is plenty for gaming. Games use many things (RAM, CPU, MOBO, GPU) where someone that is into vid editing, photoshop, many many web pages, rendering video, streaming, all of that at the same time, would get an i7, (even though i5 would do it)
So to answer your question, why do people get a 150,000 dollar lambo, ever, for anything, why would you ever get it? It's sort of like that, except there will be more of a use for i7 in near future then there will a lambo but you understand the metaphor.
ciruclate + jag excellent advice
I think people generally 'over-estimate' their computing needs, may be because they lack knowledge or may be because they just don't feel concerned enough throwing away few hundred extra bucks.
Tech side of the OP's question aside, check this out form Lamborghini 670LP4's brochure (about sound of its exhaust):
"The heavy rumble of a stormy night, through the trumpeting of mighty elephants, to the roar of a raging lion."
Won't you agree that you can't something like that in a Ford Mondeo's brochure?
The answer to that is up to you on how much of a saving you want. Take a look at this benchmark http://www.anandtech.com/bench/Product/288?vs=287
While there is very small difference in most of the benchmarks but when it comes to rendering the differences show up.
Just for gaming, there really is no reason to get an i7 over an i5, there will be no performance gain 95% of the time. I went with a 2500k myself, I'm betting by the time most games are making proper use of 8 threads I'll be in the market for a new CPU anyways.