Entity behavior that exists only in Java classes is not accessible to anything but Java. So it follows that using JDO against a relational database limits the accessibility of behavior to Java programs only.
My assertion is that having entity behavior exist in the objectbase will make it accessible to all languages that can access the objectbase. For the most part, I believe this is an unpopular idea because it is not a Java only solution. Instead it's a Java and SQL solution. SQL's non-procedural nature is its strength. If an objectbase existed where the definition language and access language was strictly implemented in Java, then many Java programmer's would be very happy. But that technology would be stricly procedural and lack the power of SQL.