As mentioned before, if going the X99 route, the 5930k would be a wiser choice if you want a PCIE SSD and > 1 video card.
If we dive deeper into various motherboards, we can see this isn't the case, yet what you said keeps being repeated on forums like gospel.
1) Scenario with a lower end X99 motherboard - For example Asrock X99 Extreme 4:
5820K has 28 lanes. That means on most motherboard, it can work as 16x/8x + M.2 PCIe 3.0 x4 configuration.
16x for the 1st GPU
8x for the 2nd GPU
4x for the PCIe 3.0 x4 M.2 SSD
"*
If you install CPU with 28 lanes, PCIE1/PCIE3/PCIE5 will run at x16/x8/
x4.
What happens when we insert the M.2 PCIe SSD? The 3rd PCIe slot gets disabled - but this is for the 3rd videocard so it's irrelevant.
**If M.2 PCI Express module is installed, PCIE5 slot will be disabled.
***If you install CPU with 28 lanes, 3-Way SLI™ is not supported.
****To support 3-Way SLI™, please install the CPU with 40 lanes."
http://www.asrock.com/mb/Intel/X99 Extreme4/?cat=Specifications
Going from PCIe 3.0 x16 to x8 is a 0-1% performance hit.
In this case, on a lower end X99 board, 5930K is only necessary when running
3 GPUs or 3GPUs + PCIe SSD or if running 2 GPUs +
2x PCIe SSDs, etc.
2) Scenario - higher end X99 motherboard such as Asrock X99 Professional:
It gets better, some higher end boards can do Tri-SLI/Tri-Fire + M.2 PCIe SSD with a 5820K.
"* If you install CPU with 28 lanes, PCIE1/PCIE2/
PCIE3/PCIE4/PCIE5 will run at x16/x0/x4/x8/x0 or x8/x8/x4/x8/
x0, and
PCIE5 will be disabled.
** To support 3-Way CrossFireX™ and 3-Way SLI™ when using CPU with 28 lanes, please install VGA cards to
PCIE1/PCIE2/PCIE4 (x8/x8/x8). * If Ultra M.2 PCI Express module is installed, PCIE3 slot will be disabled."
That means it's possible to run 5820K + GTX980 Tri-SLI in PCIe slots 1, 2, 4 and still have PCIe 3.0 x4 SSD.
http://www.asrock.com/mb/Intel/Fatal1ty X99 Professional3.1/
In conclusion, if someone is buying a 5930K over 5820K for dual GPUs and a single M.2. PCIe 3.0 x4 SSD, they are just wasting $ that could be used towards better cooling, monitor, games, etc. Since some boards can even support 8x/8x/8x + x4 on a 5820K, it requires additional research to be able to discount the 5820K as well.
In actual use unless you stream video, or have to do regular encoding, number crunching or some kind of special circumstances, there is little reason to go with a cut down Xeon with a meager 3.3 GHz base. OC'ing which ramps the already high power consumption into the realm of 5 GHz FX-chips.
Don't get me wrong it's an alternative, and it's soldered as well. I'm even considering a mini-ITX X99 build myself, but at the end of the day regardless of how much money you blow on this rig, just knowing that there is a 5960 for "a cool grand", will make it feel inferior.
For mini-ITX (in the living room), wouldn't the 35W i7-6700T be a cool option too?
In that case why even buy an i7 over the i5? Another way to look at it is going from an i5-6600K to i7-6700K just adds a bit more cache, HT and slightly higher clocks. Paying a bit extra over the i7-6700K adds 2 more cores. What do you think is going to matter more over the next 5 years?
1) $110 extra to get HT + 2MB cache and slightly higher clocks
2) ~ $80-100 extra to get 2 more cores that have HT and even more cache?
You are right that if power usage is a key factor and you want this rig in your living room, don't want to OC, i7-6700k is a safer choice. Having said that I don't think people are picking 5820K over 6700K just to justify their purchase. Even for someone who doesn't own either, it's not a clear slam dunk for the 6700K for now imo. I will say that some people buying X99 sometimes have the urge to buy $400-550 boards which completely destroys the value proposition of the X99 platform. In that case 6700K will start to pull away significantly in terms of bang-for-the-buck. I've seen some people buy a 5820K with a
$550 board which is just absurd and a complete waste of $. I feel like those boards are made specifically for 5930K or even 5960X.
However difficult the choice is now with 5820K, it's only going to get worse for 6700K if Intel launches Broadwell-E in Q1 2016 and it's going to get ever harder to recommend the 6700K once Skylake-E launches in Q3/4 2016 if the overall platform premium will be just $100. I foresee Skylake-E overclocking well and running cooler than 6700K OC since it'll have solder and better binned chips. There might be a 200-300mhz disadvantage at the top speeds but from nearly every review I've read, 6700K OC is hitting 80-92*C when all the CPU is maxed out and that's on great coolers like Noctua NH-D14/15, Corsair H100/110i. These temperatures are probably fine and won't kill the CPU since Intel tends to rate them to 100*C but still, sounds like the use of TIM, no matter how good the TIM is, was a low blow again!
Maybe Intel has a cunning plan to get more PC gamers to move to the workstation platform since they make more $ selling chipsets on those boards than they would on Z170.
Also, I think a lot of gamers are not ready to discount DX12. If DX12 games come through, we could start to see 6-core CPUs provide a tangible advantage over quads over the next 5 years. Consider another scenario where you have 20-50 tabs open, a bunch of Excel/Word/PDF documents for work, and you decide to launch a game. Are you going to close all of that just to play a game for 1-2 hours? With a 6-core, you probably don't even have to think about it.
I just think a lot of budget/younger gamers are going to pick the i5 series since they are more likely to allocate the savings towards a better monitor/videocard. Someone looking for an i7 is probably not so much concerned about budget and if performing work on their computer is more likely to consider spending just a bit more for the 6-core. I think the i7-6700K is overpriced for what it is. I feel like Intel should have clocked it at 4.4Ghz or maybe allowed all 4 cores to hit 4.2Ghz on all motherboards. I guess they are saving it for Kaby Lake.