The FX6300 does not "easily" reach 5GHz.
And power draw on a stock FX6300 let alone one at 5GHz is mental in comparison to the i3, and as a result will require a larger PSU, an after-market cooler and most probably a more expensive motherboard, meaning that if you take all of that into consideration it really doesn't seem like such a great option.
And where can you go from a FX6300? to an FX8350 or some other insanely overpriced FX8350 variant which will most likely need an
even more expensive motherboard. From the i3 you can go to an i5 for example. An i5K. Even an i7K or some kind of Xeon, most likely including the upcoming Broadwell chips, and any of those CPU's will work in the same motherboard without causing avoidable calls to the fire brigade.
Don't get me wrong, AMD make some cracking stuff. Their APU's are fantastic, the GPU's are fantastic, many of the mobile and server CPU solutions are fantastic, Mantle looks like it will also be fantastic, but I really don't think that they have cracked it (or even come close to cracking it) with the new FX series CPU's. There was a time when AMD's performance CPU's were great, as recently as the Phenom II in-fact, of which I have owned several. I wouldn't be at all surprised if they ditch the high performance CPU's and focus more on GPU's as well as APU's and other similar space/energy saving solutions which could completely change the laptop/mobile landscape.
They should play to their strengths. Making high performance desktop CPU's is not one of their strengths at the moment. I'm not saying that they should never attempt it again, nonono, but perhaps now isn't the time for another stab at it.
Capisce?