Real Advantages Of Sli/crossfire?

Isn't the only real point of SLI/Crossfire to buy two top of the line GPUs so you can have graphics power that is more than what's possible with a single GPU of the current generation? As far a price goes, I figured it would always be cheaper to just buy a single better performing GPU than two lesser performing GPUs. The only other reason I can find that someone would want to have SLI/Crossfire in their system is so they could purchase a lesser performing GPU now and then wait four-five years for the price of anther identical GPU to hit rock bottom and buy it for SLI/Crossfire. Does anyone here have two midrange to high-midrange performance cards in SLI/Crossfire installed in your system (for example, two GTX 960 cards instead of a single 970, etc)? How do you defend your purchase, and why did you not purchase a single higher-performance card?