There's nothing quite so deadly as offering the media an ineffective spokesperson. What you say and how you say it to the media takes skill and experience. They can bring an objectivity that can strengthen your plans. Make them accountable PR is an inexact science. Not everything works.
Agencies are often at the mercy of irascible editors, a fickle public, a stronger or breaking story.
Often understanding why a plan doesn't work yields important information and helps in redesigning it for success. Agencies can't make promises, but they can track their ups and downs and be accountable for their work. Their process and progress should be detailed in the billing. But hey, that's advertising. In PR, getting that message across has to be engineered much more subtly.