As far as I know the Wii U USB Helper application downloads things right from Nintendo.
There was an (unreleased) proof of concept downloader (Wii homebrew application, not PC program) for WiiWare and Virtual Console games. This theoretically allowed obtaining games without paying (kinda like Freeshop was mostly used on 2/3DS before that security hole was patched server side).
Wii disc games (and GameCube as well) were never available for purchase on the Wii Shop Channel, which means there is/was no official/legal download that could potentially be abused with piracy related apps. (Only a few Wii games released as Virtual Console, but this can be considered Wii U software and should be downloadable with Wii U USB Helper)
Many Wii disc games are available for very little money, so I don't see a problem buying them -- should be done in any in case in my opinion. For those situations where illegal downloads might be morally justified (still considered illegal by law in many countries), like broken discs and/or broken disc drive making dumping legit discs impossible, there are various sources for downloading GC/Wii games. Of course providing information on this is against GBAtemp rules -- which is a good thing. But it is not hard to find No-intro cartridge ROMs or Redump verified disc images with a general search engine. This certainly does not require an extra application.