I bet we can discuss the pros/cons, but we've seen good performance gains from turning on a feature like this. I know that the connection pool bears the brunt of this, but even then, it's just wasting resources and increasing contention around the connection pool.
a) we switched to BoneCP from DBCP and saw a huge performance gain, since it was so much more efficient at
get/release of connections
b) even so the connection pool and datanuclues both do different things when it gets/releases a connection from
the pool, and this is just totally wasted cpu and roundtrips to the database
ps - we hacked our DN to enable a feature like this, and it does seem to help performance (as long as you have adequate connection pool/db connection settings). Sadly, we hacked it in such a way that we don't have a clean patch to submit. We can talk about what we did, but ultimately what you come up with will probably be better than what we did. :)