HotHardware delved into the technical documentation of AMD's Kaveri APU and discovered that AMD did indeed consider a quad-channel GDDR5 memory interface for Kaveri. Likely the company abandoned the idea because it was impratical, GDDR5 isn't bonded in sticks for easy motherboard socketing and soldering the memory to the motherboard would be more expensive and shift RMA responsibilities from the memory maker to the motherboard manufacturer.
It's also possible that this GDDR5 mode could be implemented alongside a DDR3 main memory pool. It's possible that future versions of the Kaveri APU might implement 2x 64-bit DDR3 channels alongside 2x 32-bit GDDR5 channels, with the latter serving as a framebuffer for graphics operations. (AMD references 32-bit controllers at one point in the documentation, which is why this seems to be the most likely configuration, given that four memory controllers are mentioned in Kaveri's spec).