The first steps have been promising. So far, Nintendo has wisely selected games that could all benefit hugely from added motion and waggle. Metroid Prime, already a majestic experience on the GameCube, will be made even better with the addition of Wii-enabled first-person controls: we need look no further than Metroid Prime 3: Corruption for evidence of this. Play the original version of Pikmin or Chibi-Robo, and it's clear how the Wiimote's pointing function could make those games even faster and more intuitive. Tennis was one of the highlights of Wii Sports, so a motion-based Mario Power Tennis has buckets of potential.
In all of these cases, Nintendo will be adding motion controls that feel smooth and right. I believe modified controls in these games will do more than just complement the Cube originals -- they'll actually enhance them. And who complains about enhanced versions of their favorite games? Nobody, that's who.
There's also the fact that repackaging these games will bring them to a far wider audience than before. We love our GameCubes to death, but a whole bunch of cool software got overlooked due to the console's relatively low (22 million sales) worldwide sales. The Wii is already sat on around 35 million, and Play on Wii will make awesome titles accessible to the Wii's famed new gamers. Isn't the idea of your nan getting to grips with Chibi-Robo or Pikmin (with controls that they understand) sort of heart-warming?
Finally, there's the issue of rarity. Hey, seen the prices you'll pay for Pikmin 2 on eBay recently? Of course you haven't, because you should already own that game, but here they are. Pikmin 2 has quickly become a rare bird, so increased availability is a massive plus, and there's a good chance that other hard-to-find games will get adapted at a later date. The fact that you're also getting a new and improved control scheme is just the icing on a pretty sweet cake.