I see no reason why elements of type xs:string will examine dfdl:representation at all.

They shouldn’t' care what it is, they are always "text". I should be able to specify a bunch of inter-mixed binary 
number and string elements by scoping representation="binary" over all of them including the xs:string type elements 
without having to specify dfdl:representation="text" just to avoid an error on the string type elements. 

I believe xs:string type elements should just ignores dfdl:representation (always behaves as if dfdl:representation is 
'text').

(note: If we change this then the property precedence section for simpletypes changes slightly as representation="text" 
is implied if type is string.)