re the injector pipes. they are replaced as they don't always seal on reuse. the pressure involved in a common rail engine is something huge, so a good seal is important. plus the pipes are under constant pressure, not pulses like the old diesels. tbh, i rarely replace the pipes, but ford in their wisdom insist they are changed. also, any contamination in the fuel, however small can cause the injector to jam open, leading the engine to rev out of control, and blow up. not nice.