Shamus, you're looking at this from the gamers perspective, not the Nintendo business perspective. They want people to complete games, and get through them so that they buy more games. If kids tell their parents, "mom I beat that game! I need a new one" more often, then their sales will go up. And I can't blame them, most of their target audience is either young or inexperienced gamers. (that might be a myth, but you get my point)
I remember when I used to babysit for kids across the street when I was 15, and they just could not figure out some of the simplest things on the N64. I swear, simple patterns like, dodge-dodge-attack (repeat), were beyond their grasp. Sure, they could have been exceptionally stupid between the ages of 7-10, but there still have to be many other kids out there like them. This "demo" feature will help people like them advance the game, but like you said, they won't learn anything from it. It's like the old maxim, "give a man a fish and he'll eat for a day, teach a man to fish and he'll eat fish the rest of his life."
The ethics of gaming will be diminished, but I hardly see this as a threat to gaming itself. Every time a gamer looks up a strategy on a boss fight, reads a Prima guide, or looks up how to complete a maze/puzzle, they are doing nearly the same thing. They give up on figuring it out for themselves, and move on with it. To some extent, this isn't that bad, but it does (in my humble opinion) detract from their achievement in completing the game.
