The reason for disabling the restore transaction log job on the secondary was given in the 3rd paragraph of the article.

"Once we convert the mirrored database to a log shipped database we can disable the restore job on the secondary and have the secondary available as a hot standby in case of an upgrade failure. This allows us to quickly backout of the upgrade by simply bringing the secondary online and pointing the application servers at it. Once the system is back online we can reestablish mirroring back to the original principal. If the upgrade is successful, we enable the restore job on the secondary, allow the secondary to catch up, and then reestablish mirroring."

If you are not planning to use the secondary as a backout plan for a failed implementation then you can just enable the restore transaction log job and convert mirroring to log shipping.