You're absolutely right that the increased sales of the GC aren't going to catch it up with the PS2. The PS2 has already declared victory in this gen's console competition, and rightly so since just about everyone has one. Still, these increased sales are important to Nintendo now and in the future. First of all, the increased sales help keep their investors happy and helps Nintendo make more money so they can invest in future hardware/software. Second, the more people that get exposed to Gamecube, the more likely they'll be to buy Nintendo's next console. For instance, if a person wasn't planning on buying a GC and then buys one because it was $99 and totally loves it, then that person is that much more likely to snag Nintendo's next console. Equally so, that person may not like the GC and choose not to buy Nintendo's next console. But, at this point, I believe Nintendo is counting on people buying the hardware for cheap and then hopefully let the games speak for themselves.
Also, the next gen consoles won't be coming out til 2005/2006, so there is still a lot of life left in this gen's consoles. That means developers will still be determining which platforms to develop for (if not all of them) and it's good for Nintendo to tell those software companies what their user base is to help motivate them to develop for the GC. It also encourages Nintendo to keep developing for the Cube rather than giving up on the console and focusing on the next console. Remember the lack of N64 titles in the last days of the 64? I don't want the same thing to happen with the GC. I'd like to see quality games keep coming out for it well into the beginning stages of the GC 2.
My point is that I feel that these numbers are good for Nintendo's current picture and their future picture. Even though GC will never catch up with PS2 this gen, it helps consumers gain confidence in Nintendo and will hopefully have a positive effect on Nintendo's future console sales.