Let's assume there has never been a single instance of voter fraud that could have been prevented by an ID law. Could there be in the future? Would it matter?
I think there could be. Let's say 10 people get together, each with a different polling station. They go to a station. The first person walks in gives his real name and looks at the list to find someone registered who hasn't checked in. He goes outside and reports the name. Second person walks in and gives the fake name, looks for another empty, repeat and rinse until all 10 people have voted. Then they go to the second station, rinse and repeat. That's 100 votes, 90 which are illegal. If the real people show up, they have to fill out provisional ballots and have been disenfranchise.
To make it worse, the 10th person could get a name and they could return to the first few stations later and repeat the process. Even if the name is used, they do a provisional ballot and get 9 more votes out of the station.
That's the benefit of a voter ID law, it has a small chance of preventing substantial harm. It also has a very high chance of providing a substantial benefit by encouraging people who don't otherwise have IDs to get them and in those laws that have passed Constitutional muster, by making it possible for people who had difficulty getting an ID to get one.
What's the cost of the law? People who either didn't want an ID or couldn't get one have to fill out some paperwork and make a trip to the DMV to get an ID, for free. Oh wait, more people have IDs at no cost - let's move that to the benefit column.
And there you have it 100% benefit, 0 cost. Every state should have one.