My wife is writing a book, and just got a draft back from an editor.
The editor noted extra commas in numerous sentences like this:
He looked at her closely, but all she said was, "I am truly glad we will be living in a nice, safe fort, and not one of these wild frontier towns."
The extra comma in question is after fort.
The argument was that the comma was not needed because it was separating dependent clauses. My wife put the comma there to create a verbal pause.
There were other places for which similar commas were not within character quotes (a character wasn't speaking). This may be a different animal:
Dani had begged for a day to rest some, and to catch up on the numerous small duties she'd not been able to see to during the past week.
She's gone through all of the grammar and style guides she has access to, and hasn't found whether it's permissible or not. I'd guess at least that it should be done consistently one way or the other, but don't know which way.