Thanks for taking the time to educate and elucidate on our most misunderstood profession. As you imply, PR is a very subtle, if not enigmatic discipline that can only produce measurable "results" with constant application over time.
Rare are the instances when a PR-driven initiative can achieve a client's communications goals in one fell swoop. (Think Steve Jobs's periodic on-stage media-ready performances.) But even those only succeed when backed by a substantive story to sate journalists' discerning appetities.
Moreover, as media fragments, the single boffo media event or story will lose its potency to drive the news agenda and/or "the conversation."
Your post is important in that it sets realistic expectations for clients on what we as a profession can and cannot accomplish. Sadly, however, many PR-retaining enterprises do not distinguish between quality PR and schlock. I can think of one (unnamed) agency that has boasted of its industry-leading growth for three years running, yet the product it produces is consistently substandard.
Enjoy the new year, Brian, and thanks again for the most thoughtful and useful assessment of the profession.