Problem with having weak cores, even if you have 8 of them is not all tasks can be split across multiple cores. So if you have a single thread being bottlenecked by a slow core, then all the other threads running on the other cores are just waiting for CPU to finish processing the main thread.
This is why the "just wait until games are optimized for AMD" theory that some people push around here since, well, seems like forever now, never has and never will come to fruition. More cores is great, but not at the expense of how powerful each core is.
Not to mention that the PS4 and Xbone actually only have six cores available for games, with the remaining ones being reserved for the OS. If enough games start making use of heavy multi-threading for FX (or more realistically, its eventual successor) to show any sort of advantage, Intel could likely just push hex-cores into the desktop i7 line, putting AMD back to square one.