1. Barely ever
2. No-one except in extenuating cases (see 1)
3. Because we have a notoriously intense, demanding manager and what I saw in the Chelsea game was a group of young post-pubescent lads who had apparently spent the entire previous week screaming into their pillows in between drinking red bull and snorting cocaine off laminated posters of John Terry. I have never, ever seen a Spurs side so boiled up in my life and not only was it exceptionally unpleasant and shameful to watch, it also meant we gave up the title chase and probably sealed our fate to finish below Arsenal in one fell swoop. I have nothing but admiration for Poch and gratitude for all that he has accomplished. But by the same token, judging by what I have seen from him over the years as a player and manager I would say that he probably isn't very good at calming nerves or settling anxious minds. Harry by contrast seemed superb at this, as did BMJ. Watching Poch kneeling on the floor and pounding the ground in frustration at a missed chance in our game against Stoke made me proud to see someone who cares for his work so much, but at the same time it did fill me with a sense of dread: this is the intensity that our collection of kids have to deal with day in, day out. I see the Chelsea game as our breaking point, and Poch himself all but confirmed this when he spoke after the awful performance against Soton and said that 'something had given way' against Chelsea.
Introduce a Gallas, a Pirlo, or even a Defoe into the training ground that week before the battle of Stamford Bridge, and suddenly you have a player who has the respect and attention of every member of the squad, walking calmly from youngster to youngster, putting an arm around their shoulder and saying, "Listen, I've been in this situation many a time before. They're going to get in your face from the word go. They'll try every trick in the book to make you see red. Don't fall for it. You're better than them. Now go show the world that I'm right."
It changes the chemistry completely, and does so without compromising player development whatsoever.