This is probably a separate suggestion, but supposing someone preferred to not have OpenID users answering a poll. We shall ignore the obvious "because people with accounts are better than people without them" argument, and proceed to "So what if we get slammed with a whole bunch of OpenID spammers such that it's a reasonable antispam measure"? or ...

... you know, I think that's kind of a bad idea. But I'll leave this here in case anyone else can think of a genuinely compelling use case.