Lets put it this way... when a hard drive in "on" it is spinning and waiting for the command to read/write data. To have the drive spinning constantly is a good thing for high volume applications like servers. Whiule it is spinning, there is wear on the drive. So .... if you shut them down after x amount of time it will save on the wear of the drive. I do it, cause I have had to replace 2 drives so far, and they have all had motor failure... of course it could have something to do with the chihuhua sized dust bunnies in them.. but thats another problem for another day.
Could it be hard on the disks if they were turned on and off a few times a day. Let's say I set them to turn off after 2 hours, and they turn off 3 times during the day, and I've turned them back on three times to use my computer. Would that be hard on them?