My understanding is that Metal is very similar to AMD’s Mantle, and that is a desktop shader language. The improvements would not likely be as noticeable as they are on mobile, but there definitely would still be a benefit to having Metal for the desktop, especially with Apple’s less-than-stellar record when it comes to desktop GPU drivers. Whether they’ll actually bring it to the Mac is anyone’s guess, but I assume not sooner than the next major OS X release.