Sometimes you need administrative access
"only the owner of the data maintains control of decryption keys and decides who should get access to data"
What happens if that owner drops dead, or resigns, or gets fired?
What happens when an employee squirrels away as much data as he possibly can, locked to only himself? There's lots of people, especially empire-building middle managers, who would have that reaction. It's going to be tricky and expensive to fire them without losing all the data.