Thank you for your feedback on this topic.
Based on your replies we have decided to let the "publish()" function print a warning when being invoked without the keyword "queue_size".
The warning message itself will point to the wiki page about rospy publishers for more information.
As any warning printed via Pythons warnings module it can be disabled with an environment variable.
Since deprecation warnings are not printed by default we used a UserWarning instead.
These changes will be done for "Indigo".
For future ROS distros we can revisit the decision if we should keep the Indigo behavior or change the warning into an error or change the default behavior.
Thank you,
- Dirk