This already works in envelopes, why not let it match in the letters that go with them. When would I ever want the address block on my letter not to match the envelope?

The work around we are using for this is using the editable check box, adding a a string of characters that we search and replace for in the merge document.
The envelope merge from RE no longer works but it's easier to merge envelopes in Word than to fix this manually every time we do a mailing.