Strange things happen to protiens when they are subjected to heat over 80 Celcius, they go tough, rubbery, dis-formed, etc. Eggwhites will congeal at around 65 Celcius, yolks around 80. Drop a cracked egg into boiling water and see what happens as opposed to water under the boiling point.
I learnt my lesson a looong time ago about respecting the temperature for poaching when making creme caramel (poached custard in ramekins) for a banquet. The ramekins were sitting in a waterbath, but I had the heat in the oven cranked up too high, and the water came to a slight boil, just tiny bubbles, really, figured it wouldn't matter. When they were cold and it was time to get them out of the molds, I had tiny bubbles all over the custards, looked like swiss cheese, tasted a bit different too, a bit tougher and egg-ier-- and the Chef and the rest of the brigade never let me forget either.
Haven't got my books at home here with me in the office, but I wonder how Larousse defines poaching, or how C.I.A's excellent book defines poaching