2 parts to the situation
1. actual usage of cores in real world
2. marketing/pricing
think steam market survey suggests 50% of ppl are still on 2 cores or less below 3ghz. game developers are still/will not be utilizing full multi-core potential anytime soon. many new games have similar performance even between i3 and i5.
but the difference is very apparent in cpu/ram intensive stuff like encoding video, molecular simulation software, and multi core optimized games like cities skyline.
x99 is "enthusiast" level, and have a price premium of ~300-500 bucks.
if you're referring to those $1k z170 motherboards, those are mostly marketing/branding with some additional questionable features. it's never worth it to buy those extreme thermonuclear fusion versions of hardware.
i.e. if you want to spend 1k and are looking at z170 or x99, then definitely x99. but the x99 will only be better at certain stuff, not gaming.
it's better to scale back on cpu and channel the money on a better gfx, unless you specifically need that cpu. an i5 with a gtx1070 will be faster than any i7 with a gtx970 in gaming any day.