MySQL JOIN ON vs USING?

In a MySQL `JOIN`, what is the difference between `ON` and `USING()`? As far as I can tell, `USING()` is just more convenient syntax, whereas `ON` allows a little more flexibility when the column names are not identical. However, that difference is so minor, you'd think they'd just do away with `USING()`. Is there more to this than meets the eye? If yes, which should I use in a given situation?