It's a useless feature on an enthusiast or gaming system. If you add in a very low end GPU, you can see some benefit. But any mid range GPU will be so far ahead of the onboard graphics, the onboard GPU won't be able to do anything to add any performance. For perspective, it would be like asking if my Ferrari would be faster if I added a lawn mower engine to the drive train.
The integrated GPU's are so low end that any of the 5xxx or 4xxx GPUs is so strong that it will get no benefit from the little GPU on the motherboard, according to AMD the last discrete cards to support hybrid crossfire are the 3470 and 3450.