So lets break down the math
i7-6700, $300
Cooler, $0, included in box
Cheap, but ok H110 motherboard (Asus/MSI/Gigabyte), $60
Total = $360 for a 4C/8T @ 4GHz Turbo
i7-6700K, $340
Cooler, $10, Stock Intel HS/F
Cheap, but ok H110 motherboard (Asus/MSI/Gigabyte), $60
Total = $410 for a 4C/8T @ 4.2 GHz Turbo
i7-6600K, $240
Heatsink+Fan, $30
Cheap, but ok Z170 motherboard (Asus/MSI/Gigabyte), $100
Total = $370
You can probably get low 4GHz with a $30 cooler. Which at this point, you're still slightly behind the i7's. If you want to get it to 4.5 GHz or so, you'd probably have to get a >$50 cooler, so you're around the $400 mark. At which point you'd be pulling ahead in ST, but still lagging behind in MT.
As for motherboard quality, you probably won't notice a difference unless there are specific features you want (more USB 3.0, USB Type C, better Audio/LAN, etc). You're not going to hit a O/C wall unless you're going for some exotic (>$100 cooling) overclocks.
So I think from a practical perspective, spending the extra money on a mild i5-6600K overclock isn't really worth it, compared to going for a i7-6700 stock build. If you do more exotic solutions, the marginal increase in price compared to the i7-6700K is less as well. I guess some people just want the experience.