In a crisis situation, your physical presence can say just as much as the message you are delivering.
For most PR pros preventing leaks is just part of managing an M&A process. Communicators also are responsible for convincing stakeholders that the merger will bring added value to the company and, if the deal is rejected (read: Comcast-Time Warner Cable), where the companies goes from there.
Communicating crucial details with the public is a primary concern when a tragedy strikes. Philadelphia Mayor Michael Nutter has proven a largely effective crisis communicator.