What is wrong, if a jury discovers 'in facts which ought not to be before them good reason for deciding what verdict to return'?

Question

_{Source: p 97, The Art of the Advocate (1993) by Richard Du Cann QC.}

  If the witness must not be led, he must be guided. The evidence is given responsively, in answer to questions, not spontaneously, and the advocate must keep his witnesses under control. Twice during the trial of Seddon the Judge intervened to rebuke counsel for the prosecution (Rufus Isaacs) for allowing the witness to go on after he had answered the question he had been asked He told one witness:

JUDGE: These people are being tried on a capital charge. It might possibly be, if you do more than answer the question or make any remark, you might be doing or saying something which would not be evidence, and then the trial might have to be begun all over again.

  This objection has greater importance in a criminal trial before a jury than before a single Judge sitting alone. The lawyer, by the time he becomes a Judge, has considerable experience of [1.] putting out of his mind irrelevant or prejudicial matter. A jury has none at all, and will sometimes discover in facts which ought not to be before them (e.g. the character of a defendant) [2.] good reason for deciding what verdict to return.

I know that 1 matters for jurors also, but I do not understand the word choice in 2. Should not the author have written 'bad' instead of good? The author's argument would be debilitated, if the jury discovers a GOOD reason, even from facts that should have been excluded, for deciding the verdict, because then 1 would be violated but the correct verdict would have been decided.
I am probably assuming that 1 matters less than the correctness of verdict.

Answer 1

For very good reason, evidence that has been gathered by the police by violating the rights of the accused may not be used. If the jury hears of such evidence, they must ignore it. If the judge thinks they can't ignore it, he or she must order a new trial with a new jury.

You would open up the doors for police breaking into anybody's homes in the hope to find evidence that they somehow in a witness statement can pass on to the jury. And since the police cannot know who is guilty and who is not (if they did, then we wouldn't need courts and juries), that means that many, many innocent people would suffer.