Mike Manuel highlights an interesting challenge for agencies - how do we remain relevant without overextending ourselves?
The core of Mike's point is valid. Trying to be all things to all people is a recipe for disaster. The challenge, however, is that 5 different companies/people might call the same set of tactics or programs different things (PR, corporate communications, marketing, marketing communications, etc.)
Rather than getting wrapped up in an unresolvable semantic (er, I mean, "positioning") discussion, agencies need to focus on articulating the business benefit of what they do for clients. This sets the context for any expansion of the agency's offerings/programs/tactics. If a new arrow in the quiver helps achieve the objectives laid out with clients, by all means it should be considered if you can execute it effectively.
This focus should make the labels far less relevant.