Spin down isn't as problematic today. It really depends on your setup and usage.

If the stuff you access often can be cashed to SSDs you rarely access it. Depending on your file system and operating system only drives that are in use can be spun up. If you have multiple drive arrays with media some of it won't be accessed as often.

In an enterprise setting it generally doesn't make sense. For a home environment disks you generally don't access the data that often. Automatic downloads and seeding change that.

Is there any (semi-)scientific proof to that (serious question)? I did search a lot to this topic but found nothing...

(see above, same question)