I think this is one area where a direct relationship is worth its weight in gold. There's no secret to who media contacts are and modern technology enables you to keep them up to date of your release schedule yourself - paying a third party ensures your game will get lost in the midst of a slurry of others that they represent. If you could get a genuine honest-to-gosh PR body onside that would be different, but there's more to PR than CC'ing contacts, so paying someone to do only that strikes me as a waste of cash.