Also Mantle is supposed to be available for Linux boxes soom(tm), which is a nonneglectible thing considering SteamBox, etc.
DX will never be fully on *nix, neither will DX12, so Mantle is a good way to extend the game scene over to *nix machines - like the SteamBox and for example the Humble Bundles.
DX12 is incompatible to DX11/10/9 just like DX11 is incompatible to DX9. Games engines have to support both DX9 and DX11. And because these are incompatible to each other most games settle for DX9 because this also runs on consoles, making porting easier.
So implementing a render engine/path for DX12 will be about the same amount of work as to implement one for Mantle.
DX will never be fully on *nix, neither will DX12, so Mantle is a good way to extend the game scene over to *nix machines - like the SteamBox and for example the Humble Bundles.
DX12 is incompatible to DX11/10/9 just like DX11 is incompatible to DX9. Games engines have to support both DX9 and DX11. And because these are incompatible to each other most games settle for DX9 because this also runs on consoles, making porting easier.
So implementing a render engine/path for DX12 will be about the same amount of work as to implement one for Mantle.