Not sure builders are the best advisors on that. Sound is a major benefit. But also it you have a cold room or cupboards next to the room you are heating, then you get heat loss, unless you are heating both rooms. We didn't put insulation in he internal walls and I do regret it because you can easily hear sound from rooms on the other side of the house, compared to a house I lived in that had full insulation, and it doesn't retain heat as well if you are just heating one room, or parts of the house. But if it is a spec home you are unlikely to find interior walls insulated because it is an additional cost.