My apologies samuraiwahoo, I didn't read your entire thread. Usually people are asking why a 3rd/4th String player is being used instead of their starting HB. Depending on how many HB's you have on your team, your FB is typically your 3rd, 4th string player at the HB position.
If you like you can plug in higher and he'll actually be used as a HB, or you can set your subs such that your HB doesn't get as many carries (will require some customization / testing on your part).
If you want your HB to get fewer carries try 60 / 80 or 50 / 80 and your starting HB will get tired more quickly, thus the other players in line for HB will be utilized.
I don't know of any way other than that to get the team to just use the FB when simulating though.