Perhaps it should be a requirement for GA pilots to have passed their R/T test before being allowed to make a practice call? Won't eliminate all the poor R/T referred to, but might help.
Having fairly recently visited D&D (on a Sunday, good WX), I was surprised at how very very little R/T of any kind came through on 121.5.
On the same visit to West Drayton I was able to sit next to a controller who was having to juggle heavy metal around a serious CAS infringement at Stanstead. While it was ably and professionally dealt with it, so wasn't an "emergency", but a training fix or practice PAN on 121.5 might have avoided the situation entirely.
SD