We had set up jManage to monitor 416 WebLogic instances but due to the connection and memory leaks it wouldn't scale. We ended up creating a new property in jmanage.properties to disable the DowntimeMonitor. Now it only opens up a new connection when one is requested (we don't use the graphing or alerting features, just the command line interface). Some of the connections still don't close properly but it's much more stable now.
It would be nice for jManage to use a configurable connection pool for the DowntimeMonitor so it doesn't open up a new thread for every application. That with a scheduling queue like what Nagios has would make it a much better app suitable for large deployments.