From what I understand, in general, the only reason the defendant should testify is the defense team believes (s)he will be be convicted otherwise. It's a last-ditch attempt to gain credibility for the defense's case.
I'm sure that that's the attitude of the typical (US) defense lawyer.And it's understandable because,as Dershowitz *himself* once acknowledged,most criminal defendants are,in fact,guilty as charged.But if I'm on trial and I am,in fact,*not* guilty them I'm testifying.I wouldn't fear the prosecutor.I'd answer his/her questions plainly,calmly and respectfully...regardless of how piggish he/she is to me.By my answers...and my demeanor...I'd make even the most skilled prosecutor look,at best,foolish...if not down right evil.
What I've just said only applies if I'm innocent.Given that I'm a very poor liar and an even worse actor I'd never be a match for *any* prosecutor if I was guilty...so I wouldn't even try.If you're ever a juror at a trial where *I'm* the defendant you'll know I'm innocent if I testify...and guilty if I don't.
An example...from fiction.Shawshank Redemption...opening trial scene.The defendant,who is *not* guilty,as we learn later,is asked by the prosecutor..."hmmm...,you claim not to have done it but when asked to produce the gun you've spoken of to confirm your story it cannot be found.Now isn't *that* convenient?!" His response,delivered firmly yet calmly,"given that I'm not guilty I find it decidedly *inconvenient* that the gun cannot be found".
Yes,he was convicted...but he would have been convicted anyway.