For a useful exposition of why PR should embrace blogging, read Trevor Cook's Corporate Engagement.
... our existing clients are starting to look around for expertise in social media (and) if we don't have genuine credentials in this area they'll go somewhere else. We've already done paid work for about six clients on social media related activity. So even at that level its much more than one new client.
What's more, the double benefit I was referring to I think was the fact that PR blogging is a great way to keep building your professional knowledge, which is important for all the reasons above.
Trevor was guesting on an Australian TV show and put up a transcript in which he said blogging blogging "certainly helped us win at least one client."
My comment was:
One client? If a bottom line evaluation of the return from one of the best PR blogs really were just one piece of business it would not appear to be a very wise investment of time and resources.
OK, I deliberately bowled him a soft ball just to see how far he could hit it... but it is hard to disagree with the answer.