I suppose the concern is this: it creates a tremendous danger of wrongful convictions if a person can come forward and not testify publicly—not publicly in the sense that their name is going to be published in the paper, but not testify in a way that the accused can identify who the person is.
In order to challenge a witness you have to know something about them, right? You have to know what their possible motives are, right? Why is the person here before the court? What's this person's background? Has this person lied to people in the past? All of these things are naturally important to understand.
The difficulty is that you have a judge put in the position of having a secret hearing in private, alone with the prosecutor, where the prosecutor explains why everything's necessary—and no doubt they'll think that's true—but with no one on the other side who can explain why this is not the case, why this witness's history is relevant, why there may be dishonesty in the witness's past that is important to know.
As I say, it is as fundamental as any right in the Canadian law that a person know the case they're facing, and that includes knowing who the person is who's accusing them. How can you respond to an allegation when all you know is somebody—I won't tell you who—says you did x three weeks ago at this location. How do you respond to that? There's no way to do that.