The last time I considered this was with a collegue who is a magician/mentalist and who was also trying criminal cases. At that time, maybe ten years ago, our concern was that any good the trick might do you would be outweighed by the opposition either: a) trivializing you as a "birthday party entertainer", b) suggesting that your trick was exactly why you could not be trusted to tell the truth, c) deflating your point by exposing the trick, or d) painting the tactic as a sign of desperation, i.e. a last resort when the facts and the law aren't in one's favor.
Of course, if you're the last one to speak to the jury, then a lot of this can be avoided. And there's something to be said for showing the jury that the Court holds you in enough regard to allow you to try something novel. You wouldn't want the Court telling you to knock it off in the middle of your presentation, though.
Perhaps Mr. Leventhal's success with this signals a change in the public's regard for magicians in general? His opposition did not seem to think that they could sufficiently stigmatize him by calling him a "magician".
I suspect that Mr. Leventhal's role as the attorney representing the insurance company/defendant also has something to do with it.
BTW, I don't do many criminal cases, but it would seem to me that the judge's concern might be that if he allowed the defendant's attorney to do something like this, and the defendant lost, he might be handing the defendant grounds for an appeal.
Finally, I should note that if you do decide to do the bill switch in front of the jury, you might as well use a gaffed bill. You're not giving anything out for examination, you're not going to borrow the bill, and the jury, at least, is not going to heckle you.
Personally, I've always liked the idea of slush powder in the little paper cups they have for water.