Necessary to use a checksum on a backup and not solely the 'verify backup' option?
From MSDN


  CHECKSUM Specifies that the backup operation will verify each page for
  checksum and torn page, if enabled and available, and generate a
  checksum for the entire backup. This is the default behavior for a
  compressed backup.


All the verify backup option does is confirm that the header of your backup is readable, nothing else.

Neither of these options will tell you that you have a good backup! The only way to confirm that your backup is good is to restore it on another system