Renting at the start of a console's lifespan always made sense to us (and the N64 kid): at a platform launch, all games carry a premium price tag, so borrowing your games is plainly a good idea.
However, the Wii is now well past its second birthday, and cheap games are absolutely plentiful. Renting therefore seems redundant, but a quick Google search will reveal plenty of places willing to sling you a Wii game for a few bucks, so somebody is borrowing them.
Are you one of those people, and if so, why? Maybe there's a good reason to rent that we haven't thought of. Are current economic woes causing people to explore cheaper ways of playing games?