There was a very interesting point in Henry's book that bears on this discussion of team talks.
After Henry's first Tri-Nations in 2004, the captain Tana Umaga came to him and asked him to stop doing pre match pep talks. According to Umaga, they were completely wasted on the team who were already determined enough, and needed to go onto the field calm and prepared to do their jobs, rather than all raaked up and out of control.
Henry acquiesced to the request and stopped giving firey team talks. In fact he claims that he hardly says anything on the day of the game, and even at half time if his team was losing he only spoke with a calm and collected air.
Maybe these inspirational speeches are a bit of a waste of time.