The more FPS you generate the more drawcalls your CPU is required to push to feed the GPU, and this scales lineair, this means a stable 120/144/165/240FPS on 1080p or 1440p needs a far more potent CPU than you would on 1080P/1440P/4K 60FPS.
And with SLI I've read it could add additional CPU driver overhead so your CPU becomes even a more important factor here.
 
Many people argument that SLI has become garbage also because of scaling percentages, but is it really?
 
The videocards have remained to increase performance a lot with every iteration and architectural change, either due to optimisations in saving bandwidth aswell far more efficiënt architectures.
Yet processors on the other hand remained marginal in performance increases with every iteration/architecture since the Nehalem i7 CPU's the past 8 years. (which is a lot of time, though Skylake did a fair bit better than usual)
Comparing an i7 1366 socket CPU at the same clockspeed as a 6700K is going to show a lot less of a difference than a GTX200 series GPU versus a GTX1000 series GPU.
The new GPU series have so much more GPU power versus the old ones that the CPU's cannot keep up where the GPU usage starts dropping, though the GPU is capable of more, the FPS rates they could push out at the same settings have increased enormously.
 
Just because current CPU's lack the potentional of feeding these GPU's the SLI scaling is going to be a lot more disappointing than you would've seen back then, with often 90% or more scaling, however this is more of an issue on lower resolutions/settings and/or games that lean a lot on the CPU power.
 
The only downside is that the support of SLI seems to have declined, lower framerates have annoying framepacing issues (40~ etc), or there are other visual bugs/artefacts happening in games, or require tweaking for optimal usage.
Plus there is the cost, would you throw much more money at having more performance at the same architecture/technology or rather save it up for the next generation GPU's? (this matters on how often you upgrade too)
Let alone noise, power and heat.

thanks bcavnaugh I didn't notice gow4 added sli.

I have sli just didn't load the game or pay attention to be honest.