^^ That is the reason AMD has brought Mantle into the pricture. Tell me how many games actually use an Extreme Edition 6 Core Intel Processors for their rig, even quad core i7 is also relatively rare. Most of the gamers relies on their unlocked i5 processors. Now before mantle, by going with low or medium range i3 CPU,s though you can save money for getting a better cards, under DirectX, those processors bottlenecks the high end cards by a good margin and that's why Mantle is coming to rescue. It enables the low and mid range CPUs to handle the higher end GPUs very efficiently.
In Battlefied 4, @ 1080P resolution with medium setting, when a 7870K is paired with a R9 290X, the FPS jumps from 100 FPS (DirectX) to straight 160 FPS (Mantle). This is a huge improvement. So consider the case when of tight budget and still you are forced to buy a i5 4000 series processor by spending 12K which cut sort your Graphics card budget because you know a i3 4100 or FX-6300 might bottleneck your planned 280X. Now with Mantle enabled games, FX-6300 can provide a hell of a performance due to far higher level of CPU optimization, enabling it to handle more Graphics card draw calls, hence providing superb performance.
BTW, here is the Mantle performance review link: *linksredirect.com/?pub_id=54CL36&url=*www.guru3d.com/articles_pages/amd_mantle_preview,1.html