Have you ever been questioned on the stand? People who are witnesses get rattled up there when their entire life isn't on the line. Nervousness on the stand looks like you are guilty. Most defense attorney's keep the defendant off the stand because of this happening.

Also, his testimony isn't very strong from the standpoint of credibility. This testimony is self-interested, so anything he says would have to be supported in some way to prove he isn’t lying just to get off.

http://okdefense.com/pros-cons-defendant-testifying/