This depends on what you play game wise.
This game taxes an i5 2500k to like 90 %
Battlefield 4 on ultra setting on 64 men server on 2600k uses like 70 % cpu wise. The frostbite engine use an entire core for sound processing if you use the best quality sound.
Planetside 2 is another one that begs for an i7 in situation where there are 100 of people on your screen.
A lot of new games are extremely cpu hungry. The real game changer is partical effect, hbao and shadow rendering in newest games. They are mostly calculated via cpu power before being render by the gfx.
If you plan to play new games on best setting u wil need that extra power of the i7.
Check out the cpu bench mark for newer games i5 are starting to fall behind.
You should compare current gen CPUs for a more accurate picture.
Actually, most games don't account for hyperthreading, but do want a 3.5 GHz base clock, so the i7 doesn't benefit a lot unless it's clock speed is higher. There are a few cases where you are correct, and there will be more cases in the future, but as of right now the i5-4690K will do just fine. The i7-4790K will do better simply because it is 500 MHz faster.
You are correct, though. There are a lot of situations in games that tax CPUs a lot. I find that I am not a good host in DAMP simply because my i3 can't handle it as well, even if my internet connection is good enough.
Aside, this goes out the window with video processing. Most programs, even Windows Movie Maker, can make use of hyperthreading.
I should not that many with $300+ dollars to burn on a i7-4790K wanting it for pure gaming would be better saving for a bit and getting the i7-5920X, boasting 2 more cores and supporting four-channel DDR4 RAM and native 40-lane PCIe. It's honestly for enthusiasts at that price, but you get a monster for half the price as flagship Intel Extreme CPUs.