Granted that there may be benefit in "practice pan's", but I do not think it outways the importance of 121.5 being totally reserved for urgent/emergency tx's.
In my experience, it is only the UK that does this.
These ground tx's have a huge airborne footprint, covering some of the busiest airspace in the world. Airspace where freq. changes are frequent and where the tollerance of aircraft out of comms has to be short. The amount of "chatter" on guard seems to be is increasing, so is the frequency that fighters launch to interrogate us. The unthinkable has not happened yet, but "guard" really could be a crews saving grace.
There is no excuse for not having a good listening watch, but volume up, volume down, volume up, volume down, well, we are only human.
We all teach our kids about the use of 999, 911 or 000 etc. calls, but I do not imagine anyone would actually make practice calls to them to demonstrate what services will be at hand.
IMHO opinion any tx on guard should have us all scrambling for the volume knob to listen more carefully, not sighing and turning the comm volume down again.
BB