below is the extract from oracle Announcement!
We recommend that customers discontinue setting cursor_sharing = SIMILAR due to the many problematic situations customers have experienced using it. The ability to set this will be removed in version 12 of the Oracle Database (the settings of EXACT and FORCE will remain available). Instead, we recommend the use of Adaptive Cursor Sharing in 11g.
A number of customers have seen an increase in the number of child cursors since migrating to Oracle Database 11g Release 2. This can lead to many problems including complete CPU saturation of a machine requiring a database instance bounce or general database performance issues in the form of waits on mutexes and 'library cache lock'.