Assuming good OL blocking...it's nice now to have the hammer (Smith/Isaac) and then throw in some quick hitting guys.
I am old enough to remember Bo and his style. The fb was to Bo to what the read option is to RR. It was to his core being and Harbaugh really uses the fb on so many different levels. Ask someone like Arrowhead Blue how important the fb was to Bo.
Harbaugh uses the fb more than anyone in college OR Pro football. In addition...just like Bo....Harbaugh utilizes the TE. They are so similar in their offensive strategy. Harbaugh must have been indoctrinated with this philosophy since he played QB during the Bo era and if you recall Harbaugh used to throw to Kattus all the time.
Harbaugh is sort of the new and improved 3.0 version of Bo system. What I am saying is that Harbaugh sees the TE and FB as the equivalent to the wr and rb in value to his team.