I don't get why mantle even matters. Either it will be a success somehow when it excludes at least half of the PC market and nvidia will make its own system or it will have to be extended to nvidia to make it worthwhile for both pc gamers and developers.
AMD Mantle is open, others can use it. At the moment it only supports AMD GCN Cards. NV and take it if they want to, which would make their competition look better. The API is the same from the PS4, Xboxone, and WiiU, so the work took to make a game would be far less. if nv took it