I've spent a long time working in airport ATC and ground ops. In most/many parts of the world, if the pilot declares an emergency, either using MAYDAY or PAN, or just mutters something about having a bit of a problem, the controller will implement the Emergency Response Plan. Each plan is different and the level of response, both on the airport and at other sites such as hospitals, will vary to suit the local situation. Although a specific level of emergency is defined for certain situations, it may come down to an assessment by the controller of the information provided by the crew. From an aircraft in the air, 'Roll the trucks' is of little value to anyone, in almost every case any RFFS vehicles on the airport will be alerted and moved to one or more positions close to the runway. What is far more important, as others have said, is to know the nature of the problem and any special handling that may be needed. But I guess if you just say 'Roll the trucks', all that important stuff can be sorted out later....