Here's an interesting perspective:
The way in which most of the Anglo-Saxon states get at the truth under the law is by advocacy of particular and often opposing cases. Two views are put, with passion and eloquence, by barristers. Public relations people are secular barristers. Journalists are the secular juries. They are at least as imperfect in that function as PRs are in the advocacy one, but they are what we have. A lot depends on two things: first, on the media’s willingness to consider the cases made; and second, their scepticism about the cases made, and their ability to test them against counter claims, and to moderate them with other information.
It's taken from a useful piece by John Lloyd in the Financial Times headlined The Truth about Spin and discussed by Neville Hobson. Neville takes the argument forward, suggesting social media has a role to play allowing politicians to speak directly without the filtered spin from PRs and journalists.
...most PRs and journalists add little value to truth-telling if they’re nothing more than channels or conduits who distort and manipulate the original message. ... If you want to get close to the truth, cut out the middlemen. Let the citizens make up their own minds as to what is truth.
This is all very well, but it disregards two important functions, one of journalism and one of PR.
Journalism aggregrates information from a variety of sources and orders it a way that is interesting and enteraining for the reader; this takes time and skill. To expect the average reader to go to original sources is akin to suggesting we plant, grow and harvest all our own ingredients before preparing lunch - fine, but most of the time I'd rather pick up a well-made sandwich.
Likewise, one of the most positive roles a PR can play is interpreting and explaining sometimes dry and complex messages for a wider audience (including journalists), and often, certainly in business, on behalf of people who would struggle to express themselves in a lucid and engaging manner.
Neville puts forward David Miliband as an example - he is a skilled communicator who knows as much about the presentation of messages as most PRs. But do we really learn a great deal from his blog? And is it of any interest to the person on the proverbial Clapham omnibus?