The common charge they both face in this trial is of conspiracy. The Prosecutor gave his reasons for raising the issue - not out of prurience but rather because an affair points to how much they trusted in each other, how much they confided in each other, and that informs the charge of conspiracy.
"Mr Edis told jurors he was not revealing the affair to deliberately intrude into the pair's privacy or to make a "moral judgment".
"But Mrs Brooks and Mr Coulson are charged with conspiracy and, when people are charged with conspiracy, the first question a jury has to answer is how well did they know each other? How much did they trust each other?," he said.
"And the fact that they were in this relationship, which was a secret, means that they trusted each other quite a lot with at least that secret, and that's why we are telling you about it."
So - no, I doubt the case will hinge in this matter at all, but it is important fot the public to know.