Last year - in our most impressive sales win for OID - we won out in a competitive situation by demonstrating best performance against their customer database (100 milion users) by being faster on less hardware. There are definitely benefits to having a decade of experience of optimizing OID to run on the Oracle Database.
I'd also mention that once they (and others) get to see what we're bringing out in 11g for our new directory services UI - they're going to realize we're the easiest directory storage server to manage & configure as well.
Without further ado - the 100 million user benchmark is here.