Threads like these are fun with console lovers who never bothered to research PC hardware start to spout shit about who needs it, no point, etc. (Not saying that's everyone who likes consoles, but there's already a few posts like that)
There's a few reasons for cards like this. For example, some people want to play on huge screens. I'm not talking about a 40" 1080p HDTV, because the image quality on that is far worse than a 24" 1080p monitor (read: pixel density). Just the other day I saw an amazing PC this guy is building. 4 GTX 580s in quad SLI, to power 3 30" 2560x1600 screens. Granted, he claimed to already have spent over $11000 on it (including a $1500 CPU) and I think it's excessive, but, that's the kind of person who wants these cards.
Also, the point of a dual GPU card, as opposed to the cheaper and faster option of doing, for example, two 6970s or two 580s, is twofold (although related). PCIe has bandwidth. 16 lanes of bandwidth being the highest. For a lot of people, when you put 2 cards in SLI/CF you wind up with them each getting only 8 lanes of bandwidth (the i5 760's P55 chipset does this, as do most of the new Sandy Bridge i5 2600 P67 chipsets). 8 lanes will bottleneck the top end cards like 6970, 570, 580 etc. So it's possible that a dual gpu card on a single 16x lane is faster than two cards on 8x lanes. Then there's quadfire/quad SLI. You may not have 4 PCIe slots at 8x/8x/8x/8x - or if you do, again, you could just use two lanes at 16x/16x. So like before, the single card using more bandwidth can maybe be faster than more cards at lower bandwidth. And with certain $300 P67 motherboards, they do actually allow 16x/16x thanks to the NF200 chip, so there's another place that the two dual gpu cards can reign supreme.