Yes, I think it makes sense. We have this on our roadmap (jboss-head/cache/docs/design/JBossRoadmap.txt, search for soft/weak) for 2.x.
We also have a memory-based eviction policy planned.
Thanks for the answer.
Do we agree that this is something that must be done within JBossCache itself?
What I mean is that I do not see how someone could implement such mechanism using only the current eviction policy plugin mechanism (or may be I can, but ain't pretty .....).
I did not find the roadmap (I am browsing the CVS tree through the web interface and jboss-head seems empty: http://cvs.sourceforge.net/viewcvs.py/jboss/jboss-head/).