There's some truth in Paul H's thoughts. As I said before I've never felt bored in the training group - you don't have the time to.
But how many people ride in the TG? A handful. And I think an accident involving traffic on the M23 (which would be difficult to avoid) would potentially be a lot nastier than riders clashing on a small road due to inattention or poor riding (which could be avoided).
Watching the TT on Sat I was astonished at the amount of traffic on that road at that time of day. I don't remember it being the same last year. Personally I wouldn't have wanted to be out on the bike in TT mode that day. Can see why there are so many TTing accidents.