Others will know more about how FreeNAS manages power than I do, I want availability and stability and that has a cost. You want to shut your desktop off at night, cool. I won't.
I view FreeNAS as a SERVER. Every sysadmin I know clenches when they have to reboot servers. Hell always begins with a reboot or a crash ;). Give it pure, clean, power and leave it alone. That said, I do have a specific set of drives that I leave offline until they are needed. If you ever get hit with an encryption bug or something nasty, offline storage can save you hours and hours. That is a compromise for a specific purpose.
Guess I'm firmly in the leave em spinning camp. Even my desktop reboots only a handful of times a year. My laptop goes to sleep, but she's SSD. I'm fine with spinning them slower, slowing clock speeds, blacking out lcd's. They seem to be doing OK with the Green drives stopping and parking, but even there we have a failure mode with restrictions and they watch the counts.
As diedrichg says, the debate is decades old. On the server side, there is no debate imho. It comes down to how you view and use your installation.
Good Luck,
Mike