You can play WiiU games without it. It's mandatory for Wii, and basically all online stuff.
That's... still pretty significant, since a lot of people
want all that. But I guess Nintendo's target audience probably won't care. It's just a shiny new game system to them, not a media platform or social networking site or cellular communication service or yadda yadda yadda... I do wonder if the box for the system declares it's "backwards compatible" though. I guess they'd need a little asterisk following that statement, now.