You shouldn’t be running this command except during extreme emergencies. When you truncate transaction logs, you lose the ability to recover to a specific point in time.
Microsoft recommends that instead of truncating logs, you switch to simple recovery mode instead. That way you don’t generate logs you won’t be using, and you won’t incur performance impacts from repeatedly filling and truncating the logs. You also remove the need to regularly back up the transaction log. This has plenty of drawbacks – if something goes wrong with your database, your only option will be to restore the previous full backup. You could lose hours – maybe even days – of data.
Microsoft recognized the problems with TRUNCATE_ONLY and removed this capability completely from SQL Server 2008.
However, I'm often called in an emergency, the server has crashed because the drive filled up. So...