I think that there is some fraudulent voting ability here.
If there are 40,000 illegal immigrants in BIG CITY. Once a group figures out that since there is no actual census of these people, they create another 10,000 voters, located in spots across town. Can the illegal immigrants say 'Yes, we voted! No, we have no citizenship.'?

Since you can't quantify it so easily, then your opening the gates to a terrible flood of people who will have their votes cast fraudulently.
While I would support it locally, it doesn't set a good precedent, and I don't think it's a great idea.
While the voting/balloting procedures, I have little knowledge about, how hard would it be to have several people 'appear' at the voting station?

You can't compare or match, or whatever, since they aren't technically 'there'.