NC Rule 403 talks about how to judge evidence as to unfair prejudice. EDIT: copy of it in the legal thread.
All evidence by design, naturally, is meant to be prejudicial so no surprise there, but there is such a thing as the prejudicial effect outweighing the probative value.
Prior to allowing this testimony the judge did say it was valuable for the jury to have this information. He said this before the jury was brought in to court.