I would think the covering is there to encapsulate the fiberglass. You don't often see loose fiberglass insulation unless it was something like a DIY job of insulating a water heater with unfaced fiberglass insulation. Luckily, my current house has zero flex duct. I had forced air installed when I bought the house in 2014 and all of the duct work is metal. My previous house I had built in 2001 and for some reason the returns on the first floor had small lengths of flex duct before it transitioned to metal.