There's really only a small set of use cases that should actually benefit from this. The vast majority of organizations simply need to use IAM users, which many don't.
Really large organizations will appreciate this, IAM is not sufficient. Many account limits impact larger orgs, and having to ensure each accounts policies are set right is a trust exercise. It's much nicer to have policy trickle down from the org account (most have the consolidated billing master acct already).
