For me it's less about productivity and more about comfort. When I first started using Emacs I felt like it was a lot easier to move the cursor around and manipulate text with shortcuts consisting of keys that were already near where my hands were on the keyboard.

I use vim now though, and I find that the keybindings feel much more natural, being based on the positions of the keys rather than mnemonic-based ones in Emacs (e.g. ctrl-n for "next line").