Jan Heine points out that there was concern about pulley friction up until the Campag Gran Sport was introduced. The Simplex 543 and the Huret Louison Bobet deralleurs were perhaps the most well known of many that addressed the problem by having chain detensioning devises built in. Those of you that have ridden a bike equipped with a single freewheel or a Campag. Corsa or Paris-Roubaix will know how fee running the transmission is when you get rid of pulleys. It seems strange to me that no one bothers these days. Sure it makes very little difference to the amount of energy required to propel the bike but every little bit counts. Some years ago an American manufacturer, I can't remember who, made custom ball bearing equipped replacement pulleys and in their advertisements they had a chart showing just how much energy (friction) one would save using them.