Yeah, it is a case of how often we access the external HDD. If we access infrequently, then we disconnect power. On the other hand, recently we were accessing it often, so we just left it connected to power for about 2 months, and that is better than pulling and plugging the power because wear and damage can result at the port from that, as well, along with having current off and on with great frequency each day can help kill a device, as well.
But if I'm looking at not accessing the HDD for quite some time, I will unplug because it isn't just about the bearings and moving parts, it is also that current on a device can cause wear of it, as well. Better off removing all current to and through it (the light still is lit) for months at a time with no use. I do have other HDD's on my LAN for data back up and they are still working years later, but they will die first before my other HDD's that have not had current in YEARS. And yes, capacitors, as one example can "wear" while having no current to deal with being disconnected to all power, but in general, power itself will wear down a piece of equipment.
So, I say, do as you wish, especially related to how often you use the external HDD. If using it almost every day, I say just leave it connected to power and avoid wear and tear of the port (that have a limited life of repeated connect and disconnect), and the wear of constant power up and down, and the HDD goes into sleep mode anyway. If using it only every few months, then I would disconnect power. Just my IMHO.