Its for legacy deployments that can't update Kerberos today and can't/don't want to change behavior.
Getting the flag defined and the behavior clarified is only the first step of this process. The second is to make sure it works in the cross realm case too. The proposal I have is to make it an MUST that all intermediate cross realm tgt tickets also have ok-as-delegate flag set. It seems Microsoft does it that was and I've asked them if I've read their spec is correctly.