There have been years where I think the staff has legitimately wanted to have 3-man rotation at tailback but injuries got the 3rd man each time. Upchurch had injuries pop up in 2008 and 2009 that cost him snaps. Fowler was making a push to participate with Lacy and Yeldon in 2012 then he got hurt. By the time he got healthy in 2013, the numbers made the most sense for him at H-B instead of tailback.
I don't think their magic number is two guys. That is just how it has worked out due to the depth chart and injuries most years. They definitely didn't necessarily want to be in a featureback-spellback situation in 2015 either. But Drake struggled a bit, got injured; Henry got hot (and could take the heavy load)...and there you go.
