If you have a media failure and for some reason (such as having lost an archived log file) you cannot perform a complete recovery on some datafiles, then you might need this parameter. It is new for 8i. Previously there was only _allow_resetlogs_corruption which allowed you to do a RESETLOGS open of the database in such situations. Of course, a database forced open in this way would be in a crazy state because the current SCN would reflect the extent of the incomplete recovery, but some datafiles would have blocks in the future, which would lead to lots of nasty ORA-00600 errors (although there is an ADJUST_SCN event that could be used for relief). Once in this position, the only thing to do would be to do a full database export, rebuild the database, import and then assess the damage.

The new _allow_read_only_corruption provides a much cleaner solution to the same problem. You should only use it if all other recovery options have been exhausted, and you cannot open the database read/write. Once again, the intent is to export, rebuild and import. Not pleasant, but sometimes better than going back to an older usable backup and performing incomplete recovery to a consistent state. Also, the read only open allows you to assess better which recovery option you want to take without committing you to either.

Yes, I know it's a hidden parameter that should only be used at the advice of counsel and Oracle Support. But has anyone used it?