This is something that was actually studied back in the early 80's. Admittedly, I'm relating a distant memory from a long-ago presentation by CDC, but my recollection is that their conclusion was that disk drives are less prone to damage if they're kept spinning all the time.
There is going to be additional wear in the bearings, of course, but a disk drive is most likely to suffer damage when it's initially spun up, especially if it's been sitting idle for a long time. The heads normally float over the platter when in operation, literally flying over the surface without touching it. Only when the drive is spun down do the heads come to rest on the platter. Then, when restarted, these tiny fragile heads have to overcome something called "sticktion", and that's when failures occur.
In those days, however, it was common practice to have HDA's (head-disk assembly) stored on shelves until they were needed. Disk drives were horribly expensive (about 60 grand a pop, IIRC) so it was impractical to keep
all your data online all the time (unless you were the government or an insurance company). Sure enough, whenever I got called out to a site at 3:00 AM for a crashed drive, it was right after an operator had spun it down and back up.
As the price of disk drives came down, removable HDAs quickly fell out of favor. Nowadays, we can keep as much data as we like online 24x7. Drives have become more reliable, to the point where we just stick 'em in there and forget about them. Basic physics, however, still apply.
With those lessons in mind, today I set the spin-down rule to "Never" on desktop machines. Laptops are another matter, due to the need to run on batteries. But on a desktop machine, I don't want frequently-accessed drives spun down unnecessarily. I don't fear it like I did 30 years ago, and don't worry about shutting them off at night, but during the day they're always on.