Good use case for extraEscapedCharacters?

[Costello, Roger L.]

Hello DFDL community,

My input uses a colon to separate a label on the left side from a message on 
the right side:

Dear Sir: Thank you for your response.

By using dfdl:escapeScheme I can allow additional colons in the input, provided 
they are escaped (I'll use backslash as the escape character), e.g.,

Dear \:Sir: Thank you \:for your \\response.

Here's how I established that escaping capability:

<dfdl:escapeScheme
    escapeKind="escapeCharacter"
    escapeCharacter="\"
    escapeEscapeCharacter="\"
    extraEscapedCharacters=""
    generateEscapeBlock="whenNeeded" />

I don't think that I fully understand the property "extraEscapedCharacters". 
The specification says this:


  *   A whitespace separated list of single characters that must be escaped in 
addition to the in-scope delimiters. If there are no extra characters to escape 
the property should be set to "".
  *   This property only applies on unparsing.

Through experimentation I have found that if I set:

extraEscapedCharacters="D"

with this as the input:

Dear Sir: Thank you for your response.

then parsing followed by unparsing yields this:

\Dear Sir: Thank you for your response.

I don't see any value in that (why would I want D to have a backslash preceding 
it?). What is a good use case for extraEscapedCharacters?

/Roger