So many instances, proof of who one is has to be shown – from simple matters of getting a library card, a charge card or a membership to an organization or admission to a place or an activity. It is expected so often and people usually readily provide it. Proof of who one is in order to vote should be a requirement in all states to be consistent.
Excuses of why one need not show proof – are just that – excuses.
Proof minimizes manipulation at the polls.
Maybe one day it will come down to the additional proof of requiring fingerprints as well!