Why so much animosity? Duct work can be shown single line anyway.
Our office standard is to show all ductwork less than 20" wide as single line. Otherwise the drawing gets to busy. This is not an uncommon practice in my region. (NYC)
Working with AutoCAD MEP I cannot make my drawings conform with the office standard without going out of my way. The addition of a "duct by size" representation would address this.
For those of you who don't like it, don't use it.