Leftwich's career just got a boost

Think about if you are a guitarist and you get chosen to fill as a backup the Rolling Stones tour. That's going to be a nice addition to the resume. "Is he any good?" "Well he toured with the Stones, I know that much." Some of that magic dust rubs off on you and you are now somebody.

Same deal for Leftwich who gets to collaborate with Tom Brady. Now everyone in the nation will see him looking down at his play sheet and sending in the plays to Tom Brady. As long as its not a total disaster everyone will think some of the greatness is going to rub off onto Leftwich.
If there is success it doesn't much matter how much Leftwich has to do with it. That magic dust will rub off and he'll have a head coaching job in no time.