I had a lot of illegible writing in high school exactly for this reason. Teachers demanded writing "in pen" for the sake of contrast/readability and I gave it to them, but I wasn't about to deal with the hassle of a non-erasable method. But being a left handed writer, the side of my hand inevitably ended up blackened as I rested it on old text and smudged it in the process. So my works in erasable pen ended up less readable than the ones in pencil.