Maybe the fear centers on the possibility on how accurately and deeply such a strong A.I could identify the ugly nature of human 'values', decisions, intentions... Especially those of powerful institutions. Wouldn't it be something damning if, with high accuracy, you could get a stack trace leading up to the formation of certain human thoughts.

So, maybe the fear centers on a judgement day of sorts.. Being confronted with a dimension of 'truth' about ourselves. Terminator Judgement Day ;)