Also found this on why people don't testify:
Why would an innocent defendant choose not to testify?
The 5th Amendment to the U.S. Constitution gives every criminal defendant the right to remain silent. This means that a criminal defendant has no obligation to testify or to call witnesses. When a defendant remains silent, judges instruct jurors that the defendant has exercised a constitutional right and that they cannot infer guilt from the defendant's silence. (To read the 5th Amendment, and other Amendments in the Bill of Rights, check out Nolo's list of The Most Important Cases, Speeches, Laws, and Documents, in American History.)
But there are some excellent reasons why even innocent defendants might remain silent at trial:
•If the defendant has previously been convicted of a felony, the prosecutor may be able to attack the defendant's credibility as a witness by offering evidence of the conviction. Jurors may infer from a defendant's previous conviction that the defendant has committed the charged crime, though a judge instructs them not to. By choosing not to testify, a defendant can prevent jurors from finding out about previous convictions.
•A defendant may have a poor demeanor when speaking in public. A judge or jury may not believe a defendant who, though telling the truth, is a nervous witness and makes a bad impression.
•By presenting a defense case, defendants run the risk that jurors will find them guilty simply because they are not convinced that the defendant’s evidence is accurate. Yet in reality, it is the prosecution that has the burden of convincing jurors of the defendant’s guilt.
http://www.nolo.com/legal-encyclopedia/criminal-procedure-faq-29146-6.html
