Setting it ON introduces a high risk of index corruption in the event of a crash. Consider using delayed inserts, or wrapping multiple statements with LOCK TABLES, instead.
While it is true that corruption of the index file is more likely with this on, I would argue that it is so likely anyway that it is not worth the worry. MyISAM is not crash safe (thus Monte's Maria project), and if a server crash, all tables and indexes should be checked and repaired anyway (innodb does this auto-magically).
So can you all provide the source of this advice? When I read three google articles on the matter they all seem to agree with my logic, particularly in the face of the large performance gain.
Thanks for looking into this!