I disagree, greater-than-human-average is not useless. There's a lot of room for misinterpretation in human language. We compensate for that by non-verbal communication (posture, expression) or by asking for clarification. On top of that, most places have local expressions or idioms that are not necessarily globally recognized.

So there's two ways in which a language automaton must be better than human: it cannot rely on non-verbal hints nor can it easily ask for clarification, and it must be able to interpret many different dialects and idioms correctly -- many more than an average human would need to.