That depends on how many devs adopt the API. I'm not a fan of proprietary APIs, including physx. This will further fragment the PC gaming.
It won't fragment anything. If your card doesn't support Mantle, it will still run just the same using Direct3D (just a tiny bit slower, clearly). Feature wise i don't think there will be much difference since HW is of the same class. PhysX on the other hand, without the support either doesn't work at all or works piss poorly even on 4+ GHz quad core...
So in the end it's just a matter of developers taking time to use Mantle API in their engine. And frankly i'd prefer Mantle over anything. Extra performance is always nice where i have yet to see anything useful from PhysX other than lame eye candy that still badly degrades performance even on fully supported HW...