The Editors argue that Damian Green's passing on of confidential information in relation to illegal immigrants working as security guards and so on was in the public interest. That may be so, but it is the wrong defence for an MP to be making. The public interest defence is what the civil servant who leaks information must appeal to to justify his breaching what are otherwise very important confidentiality principles. The work of government could not go on if there were not people one could rely upon not to pass on one's provisional, half-formed thoughts or half-baked ideas that never go anywhere, let alone security-sensitive or market-sensitive information. For officials to break confidence should be considered a big thing, and they had better have a good defence for doing so.
The same should not apply to opposition MPs (or, for that matter, journalists) receiving confidential information. Provided that the dissemination of such information is not demonstrably against the public interest (e.g. we don't want newspapers publishing the cover names of our spies) - and there could be cases in which people would disagree about what was demonstrably against the public interest (I, for example, would regard it as demonstrably against the public interest for newspapers to publish the addresses of convicted paedophiles, since it would threaten order, but I am aware that others disagree with me about this) - provided that disseminating was not demonstrably against the public interest, MPs and journalists should be free to pass on information they have received, even if that information was passed to them improperly. They should not need to rely on a defence that the person passing them the information was demonstrably acting in the public interest. That is far too high a hurdle for an MP.
To take a concrete and blunt example, if the government isn't competent to hire reliable staff that don't leak sensitive information, that should be exposed by Opposition MPs. The MP should be able to say "I was told this or that, which I shouldn't have been able to find out. It was quite wrong that I was able to discover these things. You need to tighten up!" The government shouldn't be able to cover up its own incompetence by restricting the passing on of leaks to only those cases in which the leak was in the public interest. I want to know when the government is leaking against the public interest, also!