In the database field_revision table duplicates its field table even if revisioning is turned off

I noticed D7 creates a field_revision table for every field, and duplicates the content from its field table counterpart even if one has revisions turned off.

I first raised this issue here: http://drupal.stackexchange.com/questions/17364/7-is-it-safe-to-truncate-a-field-revision-table
As a side question, if this is not a bug and given I have a big text field of several GBs, would it be safe to truncate its revision table?