Mark Roques made an excellent point: God is merciful and just. The traditional view emphasises the justice of God, the universalism view emphasises the mercy of God, but the annihilation view (conditional immortality) emphasises both the justice and mercy of God.
One argument that the traditionalists point to is the unquenchable or eternal fire. However, the fire may well be unquenchable, but it doesn't necessarily mean that the thing (or person) in the fire will go on burning.
Jesus refers to Gehenna (often translated as hell in the Second / New Testament) as the place where the wicked are burnt (eg Mt 5:22, 29-30; Mt 10:28; Mt 18:9; 23: 15, 33). Gehenna is derived from the Hebrew 'Valley of Hinnom', where children were sacrificed to the god Molech, this was situated outside Jerusalem. It was the city rubbish tip where the rubbish was burnt on the fires. The fires never went out, but it does not mean that the rubbish was continually burnt - the fire did its job: consumed the rubbish.