It seems to me that the bottom line on this issue is a choice. Either:
(a) we make it difficult or impossible for homeless people to vote because the potential for fraud is present; or
(b) we make it easier for homeless people to vote and try to remain vigilant in the face of potential fraud.
Now, interestingly, in several pages of this thread I have seen no specific allegations or instances of fraud. The arguments come down to the "Oh, come on..." variety. When balancing an "Oh, come on..." risk of fraud against a person's right/ability to vote, I would sincerely hope a democratic populace would err in favor of promoting voter participation.