I've heard of 2 or 3 being replaced right after delivery of the vehicle... basically the owner found problems in the first 100 miles, and Ford realized it was a factory defect of some sort from the get-go and swapped it out.

I've never heard of a private owner needing a new battery after it has been road proven for a few weeks / 1000 miles.

I think they are expensive because Ford really does not have any to spare. Quite possibly, you taking a 2nd battery will mean one less FEH off the assembly line. It's not like water pumps where they can make 100,000 cars and 105,000 water pumps. There's just no "extra" packs layin' around! -John