This might have been true five years ago, but definitely not anymore. All new games that really need several GPUs also support them properly.
Well, all things being equal, a faster single GPU is always better than multiple slower GPUs. The upper pinnacle of performance is always with multiple GPUs, but both Nvidia and AMD have never truly perfected SLI/CF, in support, in microstutter, in scaling %, etc.
I'm not saying it's not a good thing that it exists, and that it's hopelessly broken and useless, but it definitely is far from perfect. You also have issues with needing more power, more expensive mobos, and putting out double the heat/noise from your GPUs.
Sometimes CF/SLI have higher avg, but lower minimums :
http://www.tomshardware.co.uk/radeon-hd-6870-radeon-hd-6850-barts,review-32028-20.html
I think that 6870CF is relatively close to a single 7950 in performance, but I'd much much much rather have the 7950 in that comparison. Would I turn down 7950CF though? No

But I would probably disable CF in games that didn't need it.
As for Hybrid CF with Llano/Trinity .. ehhhh. If you could mix it and combine with 7850/7870 or even 7770, it would be much much much more exciting. As it is, the best possible config (overclocked iGPU w/overclocked dGPU of highest bin) .. it basically approaches a mediocre single discrete GPU. You also need to combine it with the best DDR3 you can find to have a chance not getting penalzied heavily for low bandwidth. Llano/Trinity w/DDR3-1066/1333 = terrrrrrrrible. 1600 is okay. 1866 and beyond is what you want with a rig like that.