RE: Stored outline not being used all the times...

According to Metalink Note 132547.1, if any one of the stored outline hints becomes invalid, the entire stored outline will be disregarded, but the same note also implies (does not say this explicitly) that if this occurs, v$sql.outline_category will be NULL - which is not the case in your situation.  

Regards,
Brandon

Privileged/Confidential Information may be contained in this message or attachments hereto. Please advise immediately if you or your employer do not consent to Internet email for messages of this kind. Opinions, conclusions and other information in this message that do not relate to the official business of this company shall be understood as neither given nor endorsed by it.

--
http://www.freelists.org/webpage/oracle-l