Why is prophecy greater than speaking in tongues unless there is interpretation?
Prophecy is greater because it speaks forth the word of God with power in a language the congregation understands, so the church receives benefit. Speaking in tongues is greater only if someone interprets; without interpretation, the hearers receive nothing and cannot be taught by the message.
Les teaches that prophecy here is defined as speaking forth the word of God with the power of God, without benefit of a New Testament scripture. He points to Paul's statement that the one who prophesies is greater than the one who speaks with languages, unless he can interpret, so the church may receive anything. In Corinth, most could understand Greek, but some could understand only Hebrew or Aramaic. Paul therefore wanted them to teach and preach in a language everyone could understand, because the people had to hear the Word from men's lips and could not go home and read it. Tongues without interpretation did not give that understanding, but prophecy did.
I would that ye all spake with tongues, but rather that ye prophesied: for greater is he that prophesieth than he that speaketh with tongues, except he interpret, that the church may receive edifying.