In the UK there are Distress and Diversion cells in the London and Scottish centres that have autotriangulation capability and access to all the radars covering the FIR/UIR. Any aircraft transmitting on guard can be located rapidly using this system. It works well as all that is needed is a radio no other 'radiating aids' such as ADS or SSR. In FIRs without an autotriangulation system there is very little support to the aircraft on guard frequencies they are just there as a (supposedly) quiet frequency on which someone in distress can call for assistance. It follows that doing practice emergencies outside the cover of autotriangulation systems is of little value.
Surprisingly, despite the funny noises and back chat, pilots can be wary of transmitting on the emergency frequency and actually using internationally recognized emergency states and passing the correct information. This is especially the case with low experience GA pilots, who are also the most likely to have problems such as being lost. In consequence it makes sense for them to have a practice pan and learn that the D&D cells will rapidly assist them. The D&D controllers can rapidly shut down practice calls if there is a real emergency and pilots doing practice calls are expected to listen out and not call for a practice if an emergency is in progress.
The purpose of the D&D cells is to take executive control of any aircraft emergency in their FIR/UIR that means that usually control is left with the current controller but the D&D cell ensures that the correct agencies are alerted, that emergency responses are all coordinated and acts as one point of contact for the emergency.