The presence of godparents during the baptism of babies due to the fact that the child is not yet able to openly Express their faith in Christ, combined with God, reject Satan and all his works. That is why for the baby is doing godparents. By themselves, the godparents take the responsibility for the upbringing of the child in the Orthodox faith. They witness to God for the baby. With the baptism of adults the situation is different.
Adult may well make your own decisions about joining the Church. Adults, being in clear mind and adequate condition, they themselves testify to their faith, a combination of God and give "promise" to try to live according to divine commandments. That is why the baptism of adults is done without godparents. It turns out that "function" testimony before God for man is not matter when we are talking about the baptism of adults.
However, it is worth mentioning that some adults still like to have godparents. The Church cannot give a ban, but most people who are baptized need to understand that this practice is not necessary. Often godparents adults choose friends. The reason for this can be seen not so much religious as household. Some regard this practice as a statement of friendship.
Thus, it is possible to say that the presence of godparents at the baptism of adults is not necessary. However, those who strongly want it, can choose their own godparents. This practice does not harm the baptized, but does not carry any special meaning, making the presence of the cross in the usual formality.