They aren't used practically because they weren't designed for it.
With using a step up as a step down: You'll probably have a lot of resistance losses in the "primary." You might burn up the windings one the primary if you try to draw a lot of current.
Example 2: Using a wall-wart transformer backwards might cause the insulation to fail due to high voltages. Also, if it's unballasted (something to prevent it from drawing too much current), it might burn.