Rick has several iMacs and wants to be able to work between them, but he's concerned with multiple logins and iCloud accounts. Leo says it's a mess that Apple has created because they don't know how to merge iCloud accounts. He can share it with family sharing, but then he can't keep his own data separately. The answer is to have an iCloud account that is associated with only his login. That makes the data isolated and safe.
One benefit of iCloud Drive in Yosemite is that it's useful for photos since it's intelligent. It keeps the originals and then puts a size appropriate version of each photo on other devices.