Sometimes the argument is put forward that teaching a doctrine of universal salvation is ethically and spiritually dangerous, because some believers might take it as reason to be morally lax and not pay any attention to their sanctification. In general it may take away from the seriousness of morality and religion in general. For instance, in a discussion on universal salvation in his Eschatology, Hans Schwarz says of a sympathetic Tillich that he realizes very well that the fear that "the teaching of apokatastasis would destroy the seriousness of religious and ethical decisions" is not unfounded (p. 343).
To begin, however, it is interesting to note that my own experience has been the opposite. Knowing that Christ will eventually save all persons and defeat all evil does not motivate laziness so much as a desire to let all people know of God's goodness and his salvation. That's why I keep posting about it. As Rob Bell relates in this discussion here (start at 12:30-16:35), many people are actually more motivated to evangelize, to seek fellowship with God, etc., by the realization of universalism. The reason is why is that it's actually good news: it's not a threat, it's not a plea, there are no difficult questions to answer about reconciling God's goodness with an eternal hell, etc.; it's the good news that speaks to the burning question at the heart of human protest at evil, saying that evil will not be the final answer and will ultimately be done away with by God's power.
Moreover, a similarly puerile argument can be made against numerous other positions. Someone might object to preaching God's grace (especially under the guise of, say, imputed righteousness) because it can be taken advantage of by the weaker brothers as an excuse for sin. I don't imagine that the possibility of abusing a message of grace is reason against preaching it. More importantly, the reason is that there are independent theological reasons for not abusing the message of grace which actually stem from it (cf. Rom 6.1ff.).
It is the fact that sin leads to death, to meaninglessness, to separation from God, and ultimately to hell -- yes, to hell -- that motivates an escape from it by any means possible. The fact that hell will be ultimately temporary does not do away with its deterrent power; otherwise you might as well argue that Christ was irrational for praying to God to take the cup away in the garden, because he's going to resurrect anyway, after all. So hell is horrible; it is described as death, darkness, wailing and gnashing of teeth, and will even seem to go on and on forever to those who are there. But God has provided us with those means in Christ's sacrifice, in baptism, in the Eucharist, in the indwelling of the Holy Spirit, in the community of believers, in the scriptures, etc. If you believe the gospel, the good news that Christ died for your sins (1 Cor 15.3) -- which is simultaneously an affirmation that sin means death (cf. Rom 6.23) -- then you do not want to sin.
Universalism doesn't undermine this. If anything universalism satisfies the soul of the person who is fighting against sin. In the first place, it is an announcement of inevitable victory. That kind of confidence and optimism, a confidence that God will bring to completion what he has started in me inevitably, is eminently spiritually empowering, in times of weakness and strength alike. Moreover, the presence of the Holy Spirit in us moves us to be the kind of persons who desire and yearn and pray for the salvation of others (cf. Gal 5.22-3; Jas 3.17). Stephen, full of the Holy Spirit (Acts 6.5), prays for the forgiveness of his murderers (7.60). Universalism, therefore, is a guarantee against the disappointment of this spirit of mercy and compassion aimed at enemies (cf. Mt 5.43-8) and all alike (cf. 1 Tim 2.4). In fact, apart from a universal salvation, it would seem the hope and desire of the saints, a hope and desire for the salvation of all persons put in them by the presence of the Holy Spirit himself, would go defeated and unsatisfied, compromising the quality of their existence in the New Jerusalem.