TEs and WRs are used to be aligned one way their whole career. Moreso, with a TE back there, the possibility of a running play is null.
The 3rd down back's main roles are to catch the ball and protect the QB, but he has to be able to run the ball.
In the Packers' case, as Ryan Grant is the workhorse, and rarely gets injuried/misses time, what the Packers look out of their other running backs is #1 blocking ability, because MM likes to use Wrs and TEs as mismatches, and #2 good route running and good hands, for the occasional flat route and screen play.