Don't get me wrong, I like PR and have really benefited from good PR and if I only had a very limited marketing I would probably spend it on PR every time.
But, if you are startup and watching your precious cash, can you really afford to be paying a monthly PR retainer that is simply not delivering? 
A PR firm has traditionally delivered many things such as a press office, distribution of press releases, press conferences for product launches etc. But the Web has changed many things and much of this value is available for free or on a low-cost as needed basis. Just Google "press release distribution" and see for yourself what is available. And why pay for press-clippings service when Google Alerts probably provide 90% of the coverage for free!
The Web has also changed the way that journalists research and collate the stories that they are paid to write. They will watch the blog feeds to see what is hot, search Twitter for comments rather than recycle and serve up the dribble in your press release. Some PR firms such as IF Communications are building specialist digital teams to exploit Twitter and blog channels etc.
One key value area that your PR firm should bring and that is hard to replace, is relationships with key journalists and bloggers. In time you may well develop your own relationships with these but this is unlikely to replace the once removed relationship of your PR.
The bottom line is that your PR firm should be proactive in delivering you measurable value across traditional and digital channels in your target media (this includes geographic targets).
Their job should also include building the profile of your/your business positioning you for speaker spots (did I mention this is the best way to network expensive conferences), positioning you as an expert for features and comment on TV, radio and mainstream press.
If your PR not doing this for you then you should probably be calling them to account, reviewing their objectives, their plan and their retainer! Or maybe you would be better off cancelling the PR contract and using the
cash to hire some measurable business development or sales resource
(oops should declare my interest - see ABOUT)