Think about the competition. Strawpoll.me and Survey Monkey allow anyone who gets a poll link to vote, comment, or interact with it.  If you don't allow people to respond, users who request feedback won't get all the feedback they could.

That's what I refer to by guest users. They may not be able to upvote for rewards, but they should at least be able to vote for poll options, provide comments, and give feedback.  But that's just my $0.02.