Since the software is on a USB flash stick, you may end up being surprised at how little you actually use the external optical drive. If you don't have anything specific in mind that you need it for, save your money and just buy it when or if the need arises.
I'm a minimalist college kid that moves around a lot, so I bought Apple's SuperDrive because it's super thin and pretty light. There are wayyy better prices on Amazon for external disc drives though, and if brand or size isn't much of an issue, save your moolah and get something cheap.