I understand the students who don't want to play a solo - they like to just play the horn and don't want to do or be anything great. It is like playing a sport for enjoyment - lots of people play tennis but not everyone plays it for competition. Their goal in life is not to excel at the horn - I have found in past experience that when you come upon these players to just leave them where they are happy and content - unless they tell you that they really do want to solo but then shy away when you give them one. I worked with a teacher in the past who thought one girl needed to have a solo, and forced her to take the solo - the girl cried ( she did want to be better and solo, but shied away from it). I think he could have handled it better by smoothly encouraging her rather than yelling at her - this has worked every time in my experience.