This story has been making the rounds lately. (I'm linking to the Hot Air mention.) The comments are interesting: many in favor of the dog, few against. One clueless individual suggested keeping the dog in another room during testimony. Yeah, that'll help.
One of the objections is that the dog in question is a Golden Retriever, generally recognized as a non-threatening, cuddly breed. I can see how people might think it prejudicial to have a dog associated with kindness and sweetness on the stand. Someone suggested using a pit bull. However, I would suggest a German Shepherd Dog, or a Belgian Sheepdog.
We are talking appearance and reputation here, not fact. The fact is, there are Goldens that will scare the bejeezus out of intruders. Another fact is that, to the family they serve, the big shepherd dogs should always be genial and cozy companions. But to the outside world - in the context of court, where one is testifying - if you are looking for a less warm-and-fuzzy effect, a big, well-trained German Shepherd dog will fit the bill nicely. Few in the jury box will perceive the dog to be "cuddly." The witness needing support and comfort will have it (most German Shepherd Dogs are fond of leaning against their trusted human companions in a confidential, friendly way), along with the confidence that the accused would have to go literally through the dog to touch her. For extra effect, put a bullet-proof vest on the dog. There. No more warm and fuzzies, except for the witness, who may be able to articulate what she could not otherwise.
One commenter in the link above did note that a dog is far better than a human supporter on the witness stand, because a dog cannot influence the witness to say one thing or another.
If it is a situation where you need the witness to feel safe enough to speak without human interference, if the dog will enable the testimony, then allow the dog.