Using detached entities may simply be a reality of your system, rather than a design choice.
Simple example is the Session Bean tier and pull objects across the transactional boundary.
Now, Toplink punts on this in that detached entities within the same JVM still benefit from lazy loading.
But were you to load it using a Remote Session Bean, then you're pretty well working with fully detached entities.
Also, if you're working with Webservices, you'll be dealing a lot with detached entities, notably in terms of merging and such.
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