I'd argue it only logically makes sense to go outside of MS to make Wii or DS games.
That's a terrible argument and there is literally no way you could ever make that point against anybody with any sort of financial know-how. More sales is more sales dude. It doesn't matter what platform they're on, or how much more platform X sold than platform Y. Platform X was going to sell A anyway, and platform Y just adds B to the sales.
And you're totally ignoring the Wii, dude. Larger installed fanbase than any of the other consoles, and you're talking about one of the single most popular developers in the last ten years. If anybody could whip the third-party games into shape, it's Bungie.