JCR-1305 Jackrabbit supports creating a connection throug a JNDI Datasource and without configuring user and password. This works for some but not all provided PersistenceManagers. Some of them - like the Oracle-specific BundleDBPersistenceManager - sets default values for user and password if none are provided in the jackrabbit config. This way its impossible to use such PersistenceManagers with the plain JNDI DS.
This concerns the following BundleDbPersistenceManagers: OraclePersistenceManager, DerbyPersistenceManager, H2PersistenceManager.
There also might be other PMs (perhaps some special SimpleDbPersistenceManagers) with similar behaviour.