First, I am not an expert at all on hard disk hardware, so I cannot add much to this.
The only thing is that I have read in some places around the Internet that hard disks are not designed to spin up/down too frequently, because they may break down (for example, Martin Hinner web page says that spin up every minute would be dangerous, and it is said in many places). I also have read more details, about that normal hard disks break after 40000 spinups. Laptop's hard disks last longer, up to 300000 spinups. For example, I have found now this link googling:
If we trust these numbers, you can calculate that for example, if you spin up 20 times a day, the disk is likely to fail after 2000 days (5 years).
As said, I do not know how credible it is, so if anyone finds good references, please, provide it so we understand the reality. In my case, I do not care much, because in my setup I do not see any point for spining down before at least 1 hour of inactivity. I access the disk as central server and I do not want to continously wait for long until getting a response from the disk. I want it sleeping only when I am not really using it.
I guess that until we find a serious research work or reference, each one will need to decide about their disks, balancing the benefits of spin down with the possible risks…