This may have been discussed during design, in which case this JIRA can quickly be closed. I was a bit surprised to see my dynagroups automatically disappear when deleting the group definition that caused their creation. I'm not sure of what other users would have expected. I think a good balance would be to have a checkbox at delete time that indicates if the dynagroups should be deleted when the group definition is removed. Alternatively, we should add some text in the UI that tells the user the dynagroups will be deleted when the resource group is deleted (perhaps in a delete confirmation or simply on the page where the delete option exists).
This bug was previously known as http://jira.rhq-project.org/browse/RHQ-320