I think actually games like Brain Training are merely a means to an end for Nintendo.
It gets people interested in their platform, and that allows a game like New Super Mario Bros. to sell numbers that it otherwise wouldn't. No way does NSMB do 3-4 million in sales in Japan without the phenomenonal success the DS has had the past year.
If you gave Nintendo the option of going back to 1994 when the style of gaming/content in games like Yoshi's Island, Megaman, Castlevania, Donkey Kong Country, Sonic, Street Fighter, etc. ruled the roost ... I think they'd be quite happy with that.
If the DS had flopped its first year ... the fact is, you may never see games like Megaman ZX or New Super Mario Bros. period, because those types of games don't sell at all on the PSP either.