The thing is - when you make an investment to attract non-games, or non hard core gamers (let's assume for the sake of argument this is the same set of people), which I think we all agree that both Nintendo and Sony (and to some extent Microsoft) are actively doing, you are going to risk losing the purist hard core gamers.
In Sony's case, they are doing so on the hardware side. Since they do not create first party games (that I'm aware of) at all, their compromise is to make the hardware more expensive to attract non-gaming buyers with multimedia features. This equates to a tax for anyone interested _only_ in gaming.
From a cost perspective, Nintendo's approach is similar, but instead of taxing the hardware, they invest a portion (for Brain Training, let's say tiny portion) of their development budget towards attracting non-gamers. True, they are not focusing 100% on the hard core's, but let's face it, that is a losing strategy and one that no one else is following at all.
DS has 'won' in the same way that ps2 won. By gaining huge amounts of mindshare by selling features (games or dvd viewing) that is popular outside the core market. This tailwind helps the hardcore gamers down the road when 3rd parties get attracted, and Nintendo and Sony are able to invest more into their product because of it's success.
I agree with your points Ian, that this will cost Nintendo some, and will be a risk, but it is important to point out that this is a risk that everyone else is taking in their own ways, and further one that has shown to be a good model for Nintendo in the ds.
Also, I agree with the other arguments, that these non-games eventually have better tie in rates than movies because in reality they are games, and they allow the new market to become comfortable with the gaming experience.