Here is the thing about there being a one way mission to Mars, it HAS to be completely voluntary. If someone is willing to do such a task and realize that they will never come back to Earth then they should be allowed to. Its free choice and people have the right to choose what they do with their lives. As long as they are willing then the idea of sending people on this mission will be beneficial to us.
I strongly believe that one person alone on a mission to Mars would suffer from depression, among other issues, that would negatively affect any jobs/tasks they were to complete on such a mission. Having at least 2 people on the mission would at least allow for social interaction and a sense of community that would alleviate those negative effects. Also, if there was only one person and that person fell ill or was incapacitated then the mission would be a failure.