In terms of content purchased from the Wii U eShop, having a Nintendo Network ID is pointless. At least for now, games downloaded from the store aren’t tied in to your account. They’re instead locked to the system itself… so it would appear.
One NeoGAF user, “DSN2K”, claims to have received access to a number of free titles after receiving a previously used console. That’s in spite of the fact that a new account was apparently set up on the same Wii U, and DSN2K says the system was wiped before he received access to it. He was able to download digital games such as Trine 2: Director’s Cut and Nano Assault Neo without paying a cent.
There are likely two possibilities here – either the console wasn’t completely, 100% wiped, or all Wii U eShop content is indeed tied to the hardware itself.