If it's your secondary machine then it's probably not worth it.
If it's going to be your primary machine or you want to run games on it then it's probably worth it. Honestly I think you'll run into Ram/HD limitations first.
One area where the i7 helps is transcoding (DVD/Bluray rips) is a bit faster on the i7 over the i5 but that's not likely something you'd be doing on a MBA.