The points for use of 121.5 for UK training calls are:
1. The UK D&D organisation wish the frequency to be used for practice calls. It proves the auto-triangulation system works on a daily basis and assists them with controller training.
2. The UK pilot training system requires its use for reasons already stated.
3. Trainee pilots gain from its actual use for practice.
4. By hearing others using it each pilot can be sure that a) he has it dialled and that b)his set is working correctly.
Against it?
It annoys some self-centered pilots, but surely far less than a busy in-use frequency.
Seems to me that the attitude of those against its use is similar to the attitude of those who want minor airfields closing on the premise of noise, but still want to travel to their holiday by air i.e. NOT IN MY BACK YARD.
If it annoys or becomes a distraction then why not use the volume switch or temporarily deselect it. Or is that too much trouble?