The scriptural instruction implies that there is an interpreter present and that he is trustworthy, yet I have heard interpreters which I did not deem trustworthy. The method described may be used for first testing the interpreters, so if these give an interpretation of a tongue or call it out for being gobblydegook (which one would presume would happen on occasion as there are a lot of charlatans out there), one can be assured that the interpreter is not just making up words from his own mind.
God clearly speaks against false prophets in the Old Testament, and one of their sins is blasphemy. Those who pretend to speak in tongues or interpret, when they are only babbling or speaking their own thoughts, commit the same sin. It is our duty to test those who claim to have a gift or word from God, and to ignore those who can't prove they are not liars (e.g. with the aforementioned tests I mentioned).