Currently, all columns in all tables are considered private.
This can be chosen on a per-column basis (whether the column can be replicated).
This allows users to calculate stats, etc. on http://quarry.wmflabs.org/ and similar tools.
X1 (where most of the Echo tables are) replication has performance issues, so this is not a high priority.
As noted, we can do partial-table replications, so we could e.g. replicate just event_id and event_type for stats.
Not sure if that would have any privacy ramifications; maybe in some cases it could allow you to deduce that at least one person is watching a page (do an action then check the replicated table).
We Don't Need This is a fine answer if we don't think it can be done safely, or is unwanted.
I'll close it for now. We can always revisit.