When I was TMSO I felt it was my duty to encourage peopleto stay on the school, and to enrol as soon as possible.
That having been said, there were a few who didn't feel they could do it, due to having a nervous disposition, etc. That was fine - I left them off.
There was nothing more disruptive than someone not turning up on the night,as it affected not only the one scrabbling to prepare a talk at the last minute, but those around them (hehehe).
In the three years or so that I was TMSO I only had four cancellations due to ill health, and only on one of these did I have to cover the material in the talk myself. Ironically this was while I was on the way out, and so I put my own slant on it