Here's my thought. If you don't pay the small fee, you get to keep the game you already have playable on your Wii U. Its up to you to decide if the small fee is worth it for the new Wii U VC interface, including the option for portable gaming, customization of what buttons do what, and more controller support (Gamepad and Wii U pro controller both also supported).
Having said that. I think its a little unfair that the games cost the same now as they did on the Wii when they were released on the VC there. Its almost like you've been a little bit punished if you already bought them the first time around.
All of this is null and void for me though, cuz I was a dirty bastard and installed wads and used emulators, so I never bought any VC titles on my Wii. My understanding is that the Wii U mode remains unhacked (at least to the masses), so should I want any VC titles with that great new ability to play them portably, I'll have to shell out the full price. Which quite honestly is fine by me.