Unless I misunderstood, I believe that you are incorrect with this statement:

I've never heard of a major browser that actually uses the DTD line (that is, render documents differently depending on the DTD settings).

I am certain I have had to set the DTD to a specific value to get IE to operate properly (at least for my definition of properly :-) ).

• At least Internet Explorer 6 changed its box model (http://msdn.microsoft.com/en-us/library/bb250395%28v=vs.85%29.aspx) based on the doctype. There were also other things that changed.
• IE 9 changes HTML5 rendering based on the doctype (http://msdn.microsoft.com/en-us/library/cc288325%28v=vs.85%29.aspx).
• See also Quirks mode.