Filler bloks would have to be 1x1, if they were to be universally useful. The whole point was what do you do when you can't add anything more, but you still have empty spaces.
I don't mean to come off as rude, but did you not read what I said? -_- If you CAN'T add anything more, because of spacing, then a filler would be needed.
Face war it is then. -_- And as an example, a filler would be needed if a module didn't exist in a shape such as: X X X