They are part of the nine gifts of the spirit. If you discount tongues you have discount the other eight.
And the "God's word is complete" theory was invented by reformed theologians who did not want to entertain the supernatural.
What a bunch of balderdash!
Apostle Paul in 1 Corinthians wasn't even talking about an 'unknown tongue' (the word "unknown" in 1 Cor.14 was ADDED by the KJV translators). The translators apparently also wrongly assumed the gibberish tongue was the true cloven tongue of Pentecost. They had no translation authority to add that word "unknown" in 1 Cor.14.
Paul was teaching about "diversities of tongues", which means known languages of the world (1 Cor.12:28). If he had meant the cloven tongue of Pentecost he would not have said "
diversities of tongues", which means various languages.
In 1 Cor.12, the gift of tongues (languages per the Greek) is last on Paul's list. He did not compare it as the best gift.