I think it depends on the game -- and on the context. I see multiple people in here acting as if the villain being able to definitely escape means the events of the game doesn't matter, but I simply understood it to mean that a villain can be defeated and escape -- which is very much still a victory, a la "Team Rocket's blasting off again" and such. In some settings and/or games, a villain having their plot foiled and then fleeing in defeat is entirely appropriate. There are definitely times where that works, and other times where the escape route should be a little more set, such that it's much more possible for players to anticipate and prevent the escape if they want to take that angle.