This is one of the only criticisms of Xcode I'd agree with. And it's a minor issue for me.

It would, but in the meantime, I find AppCode to be pretty great for that[1].

1 - https://www.jetbrains.com/objc

I immediately downloaded appcode when I found I couldn't rename variables in swift

But you can rename variables in Swift 3 + XCode 8: move cursor to a symbol and press Command + Control + E.

Sadly, that's the only refactoring support available right now.

Sadly, that only renames within the current file.