More than 2MB frame buffer is not just useful, I think - it's required for non-crap PAL support:
At a minimum of 5 bytes per pixel (24 bit framebuffer in either 6:6:6:6 or 8:8:8 format plus 16 bit Z buffer), widescreen progressive PAL with square pixels (approx 1024x576 = 589824 pixels) requires well over 2MB (2949120 bytes).
Hence, If the Wii has only 2MB embedded frame buffer, this is not big enough for PAL without tiling - which I can't imagine would be practical would it? Wii hardly has vertex transforming power to burn for early Z pass? Or perhaps Europe is just going to get shafted again, and titles will all render at 480 lines and/or non square pixels and upscaled output?