Fixed in cumin svn version 3990.
Now lists selected items in confirmation form.

To verify:
navigate to Messaging tab
click on a broker
you should see a list of queues with checkboxes
select one or more checkboxes
click on the Purge button
you should see a Purge form that lists the queue names you checked

Have seen it working a couple of times with 1.3 released version of cumin.