If it's going to be on for any length of time, it's a good idea. Drives in a computer case get some airflow so they don't normally need a direct fan, but an external case needs at least a small fan to provide a little airflow. The fan also provides cooling for the control chip and power circuity of the enclosure.
I disabled the fan in my external box, but I only have the drive on for a few hours at most, every few weeks. Probably still not an ideal situation since it of course reaches full operating temperature within that time.