Nintendo have always been the casual, lighterhearted gaming company. They created Zelda OoT and the Pokemon series, 2 things I would be lost without. And without my N64 I wouldn't even be a gamer today. I've always owned some form of Nintendo portable, because I find that they do it the best. PSP are big budget, hardcore games on a tiny screen, and just aren't suited to travel, whereas Pokemon was a pick and up and play for 5 minutes kind of game that meant that it was actually fun to play on a journey somewhere.
I know that when I want something fun to do that won't take much time to get into and won't bog me down from whatever else I was/should be doing, I can always turn to my Nintendo games for a bit of quick fun.
When I want to sit down and get really involved in a big game, I turn to Xbox and, usually, Oblivion.
If Nintendo started to blur these lines too much I'd be horribly confused. Black would be south, Up would be a desk and my door would begin to talk. The world order of gaming is pretty well set out right now and it works smoothly. PCs have their super in depth RPGs like the Witcher, the Xbox and PS3 have simple shooters and fun RPGs, and Nintendo does quick, fun games that are lighthearted and easy to get into.