Insulating Pipes

Is there any real advantage of using foam pipe insulation on cold water pipes to reduce sweating and hot water and heating pipes when those pipes will be in fiberglass insulated walls? I am seeing more interior walls insulated today to reduce sound transmission. Is it a waste to use foam pipe insulation only to cover it in fiberglass?

Once fiberglass gets wet it looses some of its insulating ability. Stopping sweating is a good idea and insulating HW pipe, espcially if it is copper, will help hold the heat in while in use and help slow cool down between uses of the same shower when taking back to back showers..