Personally I think that Sony and MS are bigger targets for piracy since they have games more appealing to, well, people who would pirate games (18-34 men).
A seven year old's mom isn't going to figure out how to pirate Lego Harry Potter on wii. The casual market that Wii targets is also the one least likely to engage in piracy, in my estimation. Wii Fit? Not a target for piracy, even if the dongle wasn't a factor.
That said, the Wii is by far the easiest current gen console to play pirated games on.
Without overcomplicating it, I think the 7 year old boys with pirate fathers account for a small percentage of Wii sales, and that they are also more likely to own and play other consoles.
A seven year old's mom isn't going to figure out how to pirate Lego Harry Potter on wii. The casual market that Wii targets is also the one least likely to engage in piracy, in my estimation. Wii Fit? Not a target for piracy, even if the dongle wasn't a factor.
That said, the Wii is by far the easiest current gen console to play pirated games on.