I would suppose that it could be used like this:
Let's say your ditto did not have Imposter. It would need to use Transform. So, with the Quick Powder its speed would be boosted, giving it a potential chance to be able to move first and transform.
With the Metal Powder, if it ends up moving last, then it'll have some extra bulk to help it take a hit.
Still, its always better to get Imposter Ditto.