It seems that most boards have the 32KHz external crystal hooked up to the XTAL pins, but most of the examples aren't configured to use it (specifically the XPlained and examples, and the adafruit feather and it's atmel START project file).
What are the pro's and cons of using one? I am currently using the OSCULP32K ultra low power oscillator and it works fine, though I image accuracy over a long deployment might be an issue in the future. The low power estimations in the datasheet are done with the external crystal, does this mean there is not a power trade off when using the external crystal? Is the main trade off accuracy and start up time?