They have a strong games library which means you could argue for and against going third party. Strong games library means you can push some hardware with it but it also means you can sell many many games if you develop for other platforms. A Super Mario on PC, XBox and PS would sell probably really awesome.
Not sure what's best for Nintendo, for me it would be nice since I really like some Nintendo games but I don't like their hardware since N64 or maybe Gamecube. I'm not a graphics whore who only plays games if they look nice but I would still prefer Nintendo's art-style on strong hardware. And all the games I care about from Nintendo would run awesome on other consoles or PC with Xbox or PS controller.