If NVidia cards get much less performance in Mantle games it means that gamers will be tied into AMD cards. If NVidia decides to create their own CUDA based rival to Mantle, we're going to be in a situation where developers take sides with games optimised for one vendor or another. How is that good for gamers?
Because developers won't do that, it's that simple. Again this is not the 90's, game dev's aren't all new to 3d gaming, aren't working on tiny budgets, neither are the gpu manufacturers.
Today gpu manufacturers spend millions on dev support and game devs themselves spend, well, up to quarter of a billion on a single game(gta 5, though I expect Blizzard will have spent way more than that on Warcraft).
The idea that Dice would create an engine and attempt to licence it out but skimp on an important API for the sake of relatively little work and making their engine less sell-able is simply wrong.
More performance means more performance, more usage of the hardware you are paying money for is MUCH better than paying for double the performance hardware to make up the difference in lacking performance.
What Mantle will most likely do is either adding Nvidia support at a later date via pushing it to a group to control it outside of AMD, or Mantle will force openGL groups hand to actually get openGL working great, and maybe DX as well.
The lock in to windows and DX has been hurting the industry for a decade, SOMETHING has to happen for that to change. Gamers being given the choice of OS to install, having a better API, better value for money from all their hardware and better games is nothing short of brilliant for gamers. If that is from Mantle itself, Mantle forcing the industry to come up with an alternative, or Mantle forcing MS/openGL group to pull their finger out and make something decent... I honestly don't care.