I know there was a lively discussion about this happening a couple of years ago with the Indiana Jones pencil sketches.
If you are erasing the sketch, then drawing your own sketch and keeping the card in your own collection, I say go for it. However, I would never erase a sketch card myself.
If you are erasing the sketch, then selling the blank, that is unethical and probably illegal as well, since the sketches have to be approved. If you go so far as to mis-represent the blank as some kind of rare unreleased artist proof card (which I think happened on the Indiana Jones cards), I hope the card company comes after you with their lawyers.