Well it depends on the game. If a game takes full advantage of the processing power of the 360 / PS3 for other things than graphics like physics, AI, etc. then it would prove to be quite difficult to cut that down for a port. But there is not only black and white. Of course there exists a middle ground for games where you have to reduce here and there, but that still can retain their basic gameplay. On a sidenote: Games that will try to push the graphics envelope on the PS3 might still be quite easy to port, because to archieve that, they would most probably need to use some of the SPE's for this, as the the GPU alone won't be superior to the 360. Still the point remains.
The thing that some of you fail to recognize though is, that this works in favor of the Wii. The GC was quite capable at its time and easy to develop for, but it did not get that much games from 3rd parties. So Nintendo decided to provide a machine, which is still very easy and cheap to develop for, and on top of this provided an unique control scheme, which gets the Wii basically free exclusives, as like you cant port heavy CPU-miracles to the Wii, you cant port the controls to 360 / PS3, unless you want to sell extra peripherals with your game. So while the Wii wont get that many one-to-one ports of 360 / PS3 games, it will get games specifically tailored for its controls, which will do me just fine. Furthermore if Nintendo wants to get their momentum going, they dont need more "hardcore" games, they need more games "for everyone" and i dont see too much of them on 360 / PS3.
And to finally comment on the original post: I think, that it is quite a respectable stance Team Ninja is taking, but not everyone can afford to do that money- and quality-wise. But it would be quite admirable if more teams would adapt that philosophy. Just think of a great game, choose the best platform for it feature-wise and forget the rest. One-to-one swordfighting goes to Wii, online-driven team-game goes to 360 and graphics and physics simulation goes to PS3. If you think of it, you could even make a lot of money with that. Decide early on a next-gen system and come out at launch with a game making perfect use of what the platform provides - strike and high sales ensured. And all those mediocre games can keep getting screwed, going multiplatform or whatever 
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