Getting somewhat political, if I want a transfer switch for my house, I'll get one! "Requiring" it sounds just like the arc fault, GFCI, and Tamper resistant outlets requirements that people complain about being shoved down their throats. People doing stupid things with electricity has been around as long as there has been electricity.
I believe that requiring transfer switches fall under electrical design not electrical safety code. Based on history, the smallest cheapest device which meets the code will get installed.
There are other things I would like to see required before transfer switches.
My $.02.
