Wii U can only play Wii games, not Gamecube, as the Wii had a Gamecube emulator within itself, from what I have heard, and the Wii U doesn't, but meh :P
I only emulate games that I own nowadays. It's just easier to do that as it means I can mess around with cheats and stuff without messing up my good saves. I don't pirate at all, I'm opposed to it for the simple reason of it being theft, which is wrong. If you spent ages doing something (regardless of how big you are as a company), would you like it to be stolen by a lot of people? I treat things by how I like to be treated, and that includes no stealing of intellectual property.
In my opinion, emulators should be used for those that are past their date. Games that are no longer in production anywhere, on consoles that no longer exist and may not be rereleased (I still would buy it if it was rereleased though, like Kirby 64 on Nintendo 64). Emulating current generation of games I would only do if I own it as a messaround save (or to record, as have you seen the price of that Nintendo 3DS recording software?!) as I would own it anyway, so I already paid the price for it.