You can disable logging in the admin as well as in local.xml so writing to the DB is completely stopped. See this for more info: http://www.axertion.com/tutorials/2012/12/how-to-disable-magento-logging-to-the-database/ . I know some large-volume sites that have logging disabled completely.
Is there any reason, other than being the admin's reference, to keep any logging (log_* and/or report_* data) at all?
Yes, there are in fact reasons that I would not recommend to disable all these.
At least the predispatch ones should NOT be disabled, i.e.
<controller_action_predispatch>
<observers><log><type>disabled</type></log></observers>
</controller_action_predispatch>
<controller_action_postdispatch>
<observers><log><type>disabled</type></log></observers>
</controller_action_postdispatch>
I did this on a production system ones and it lead to errors with regards to the user's compare products functionality.
The problem was that the pre and postdispatch logs generate IDs for guests. When deactivated, that ID is always 0 and thus, when a user adds something to the compare list, all guest IDs 0 are assigned to his user ID making the compare products functionality useless for all others.
It seems that the other logs can be disabled without impairing functionality.
customer_id it seems.
Mage_Report, which hooks onto an event fired by Mage_Log, but the problem is that guest data seem to be left alone when the cleaning takes places, leading to a ton of old and probably useless data.