The old saying goes that the cobbler's son is always the worst shod, so PR companies are often bad communicators when it comes to talking about themselves. A recent example of this is the way chris lewis of lewis pr has allegedly been screening all the derogatory comments about the means in which he has built the agency.
Blogs are the 'fresh meat' business opportunities that PR agencies think they can provide clients with 'value-added' services on (rather like webcasting, PR management extranets and internet press rooms 5 - 7 years ago). So agencies are now experts in the area of communicating via blogs (the buzz phrases to watch out for are micro media, the long tail, stakeholders, blogosphere, the viral nature of information).
As any agency would tell you, blogging is as much about developing a dialogue as opposed to one-way communication. Firstly look at chris' pristine blog post here and then the discussions about the mysteriously missing comments here. Hmmm (Kudos to Spin Bunny).