Without voter id laws, it is possible to cast a vote in someone else's name -- just as O'Keefe showed. But this is NOT how election frauds take place. In order to get enough "extra" votes to be meaningful, one would have to bus people from poll to poll, hand out new names to each fraudulent voter at each poll, and have poll workers ignore the periodic influxes of people. In addition, such fraud involves so many participants that discovery of the scam would happen immediately, No politician in his right mind would attempt fraud that way.
And while e-vote fraud is possible (and perhaps even happened once), that is also a bit difficult to manage and keep under wraps.
See second comment.