Intercept evidence

[ewx]

What's the logic behind the refusal to use phone tap evidence in court?

I'm having trouble imagining that the Home Office is acting out of a deep-seated concern for the personal liberties of the people tapped, given that their preferred alternative is to lock suspects up without the inconvenience of a trial in the first place; anyone whose phone is tapped presumably knows what is in the intercepted conversation anyway, because they were party to it; and the fact that your phone was tapped doesn't reveal anything more than that the British state was interested in what you were saying, which you knew anyway because you're being done under the prevention of terrorism act.

Protecting sources is certainly something to worry about in some cases, but I don't see how it applies to phone intercepts.