Ian, keep in mind the Gamecube wasn't really a very popular system by any stretch, so this means there are tons and tons of people who have NEVER played games like Pikmin even once. So for them, it isn't really a matter of rebuying an old game, because to them its entirely new. If you are one of those who own the old Pikmin then I can see your point, but you are in a minority. Most gamers have not played Pikmin, and since you love the franchise so much you should see how this is a great thing Nintendo is doing here.
By introducing the games to the larger Wii audience, Nintendo is promoting and building up the franchise's popularity, and this will help ensure the franchise sees more sequels in the future. And personally, I think the franchise is very well suited for the DS platform. Hopefully Nintendo will port it over there at some point.