I think it is useful for more than just 'scaling bugs'. If you want to play games running in 320x256 and have scanlines, it would require a 640x512 resolution, however this is not a standard VGA resolution, so either one would have to resort to 640x480 and cut a bit of the image or use 720x576 and get black borders all around.
However, if it could scale 4x on both axes, it would fit perfectly with a 1280x1024 resolution. This is why I could see a 4x feature as being useful. I realize that it would take time (and thus money) to implement it, and there's other things higher on my wish list, like the Vsync feature. That being said, I think the MkII is very useful as it is now, and gaming will never be perfect on an LCD monitor anyway.
Since my monitor has 1200 lines, I might as well use 720x576 for PAL which works fine for me, and probably gives as good a picture as 1280x1024 would.