What happens if people start colluding behind the scenes of arXiv to change their records? Even if the people running it now are totally trustworthy, they won't be in control forever.
Publicly tweet the hash of your PDF, email the hash of your PDF to yourself, upload your PDF to google drive, dropbox, github, and s3. Essentially the whole world would have to collude against you for someone to claim they wrote it before you.
