The tensioners and water pump should always be replaced or you run the risk of major problems when one of those extras decides to go south.
The fact that someone spins an idler or tensioner pulley by hand and it feels smooth as silk means nothing. With aged grease inside of the pulley bearings the pulley may feel fine at the time of the belt replacement and may disentegrate 5k miles later. The same thing goes for the water pump.