What's Up With That? #1: Do you swear to tell the truth?

KM was watching Judge Judy this afternoon (I know...educational television), and I found myself wondering aloud...

Why do courts — real as well as "reality" — still perpetuate the charade of "sworn testimony"? Has there ever, in any courtroom, been a witness who when asked, "Do you swear to tell the truth, the whole truth, and nothing but the truth," replied, "No, Your Honor — I intend to lie like a bad toupee"? No perjurer is ever going to announce his or her fabrication in advance.

It seems to me, therefore, to make more sense to simply begin each witness's testimony with a caution from the judge, along these lines: "You are required to answer all questions truthfully, to the best of your knowledge. Giving false testimony is a felony, punishable by time in prison." The witness is then not put in the rather stupid position of lying about lying.

Then again, I'm not really a judge; I just play one on DVD Verdict.