Answer: - It's easier for a typical homeowner to rework/repair down the road. You make your new joints with liquid adhesive, not a torch and solder. -The builder may also be calling PEX pipe plastic. While that doesn't require soldering, the fittings may be a little specialized. -if there is no difference in price then copper, have them insulate the hot water pipes. It will save you money in the long run. -No plastic is ever as good as copper. All plastics dry out over time.