I've never heard one way or the other, but I agree that it was a mistake - whether the idea was the attorney's or Damien's. Damien simply believed (naively) that you cannot be convicted of a crime you didn't commit. I can totally understand Damien insisting on testifying, and I can totally understand an inexperienced attorney suggesting it. The problem, as I've said before, is that the prosecutors were much more adept at cross examination than the defense attorneys were at redirect. That, IMO, was very telling in the jury room, and, at least in part, lead to the false convictions.
In almost all cases the defendant understandably WANTS to testify on their own behalf. I can also see inexperienced attorneys also not only suggesting it but not knowing how to talk their client down off the ledge and convince them it's not in their best interest. No question, though, that the advantage laid with the prosecution as far as experience goes. Everyone saw what happened when courtroom experience was met with courtroom experience so many years after their convictions.