For many clients, outsourced and project-based marketing communications has an economic rationale that works even in a strong economy. It allows companies to do more short-term activities without a large commitment. If a project proves successful, they certainly can lead to longer-term relationships. A prestigious address does not make an agency do better work or increase the chances of media coverage. Agencies love to drop names of contacts, but these may not be the right reporters, editors, and analysts for your company.
Experienced pros develop new relationships as needed.
Look at their clip book, but don't be too impressed, especially by clips for big name clients. See what they've accomplished for clients that are about your size and budget. The people showing you past results should be the same people who will do the actual work on your account. Your needs and budget may vary from month to month. Your provider should be able to work with a flexible budget.