I'm not saying the Diecimila shouldn't be used for a one-off production project, just that I don't want to see it cluttered with features to make it a production board.
The beauty of the Arduino for this same target audince is its hardware simplicity. Why make it one more thing people who don't have an electronics emphasis have to use to get the board working the way they want?
Personally, I'd use the board in a project if I wanted the usb/serial chip and only if I did. Otherwise I'd find it easier to solder the MCU/crystal/caps onto the same perfboard as the other parts.