I did not buy Zack and Wiki and I'm glad I didn't. I love the game but it's missing something that most good point and click adventure games have: length. I'm on day 4 of a 7 day rental and I'm not too far away from beating the game. Why pay $40-$50 for it when I can just pay $7 and enjoy it just as much. The same could be said of Trauma Center: Second Opinion yet Trauma Center managed to sell enough copies to warrant a sequel.
If Capcom wants me to buy a copy of Zack and Wiki, then they can provide enough gameplay that I won't come out ahead renting. Nintendo's first party titles tend to be long enough that it will take me multiple weeks to beat the game. There's no way I could've beaten Metroid Prime in one week. I would've easily spent more renting it than I would just buying it.
"that Wii Remote with 10 mini games is worth absolutely nothing compared to the incredible gameplay of Super Mario Galaxy, or the hardcore adventure of Zelda."
Maybe to you, but as far as I'm concerned, the whole family being able to play Wii Sports because we have enough controllers for everyone is worth far more than me playing Mario Galaxy by myself.
I'm a consumer. I buy what I want at a price I'm happy with. Right now, the used game market and the rental market are the only ones catering to me. I'm not going to feel guilty about that and if my contribution isn't enough to keep those third party developers afloat, then it sucks for them.