Why are some Nintendo fans so opposed to going third party?

[JEMC]

I think the biggeest reason is nostalgia. For a lot of us, Nintendo means videogames and consoles.

Nintendo were the ones that made videogames a worldwide entertainment. How many japanese games were launched outside of Japan before the NES? And how many american games arrived here to Europe? The NES allowed that to happen. Also, while Nintendo wasn't the first to launch a console, they certainly are the ones that have been doing so for longer. SEGA is a shadow of what they were, SNK is gone and Atari is dead (or almost). We only have Nintendo from those times, and losing their consoles would mean the definitive end of an era.

Also, as some have already said, their philosophy when designing consoles and controllers is more open than the one the competitors have shown, and don't fear to try new things. Although after what happened with the wiimote it's clear that both Sony and MSoft have upped their game, one keeping Kinect and adding rumble triggers while the other one has chosed to add a touch pad to its controller.