With NFL teams becoming more analytically-inclined, we are seeing more bad run defenses among forward-thinking teams that are happy to allow small gains on the ground between the 20s, and we are seeing more offenses develop running backs who can put pressure on the defense in a variety of ways: running between the tackles, running outside the tackles, running routes out of the backfield, and even going in motion and/or lining up in the slot (2017 rookies Alvin Kamara and Christian McCaffrey each lined up in the slot on over 25% of their snaps!). Even before these growing trends, running backs were more valuable than wide receivers in DFS from a “floor” perspective because of their guaranteed touches and their scoring potential — but with full-PPR scoring on DraftKings and FantasyDraft, and with half-PPR scoring on FanDuel, this NFL movement away from the “disposable running back”/“split workload” trends of the mid-2010s is making these “Multiple” backs (RBs who can put pressure on a defense in multiple ways) more valuable, and more readily available, in DFS.
There are no “every time” rules in DFS, and it is dangerous to slip into a thought process in which you act as if there are. But it is currently advisable to use your FLEX spots on running backs the majority of the time — on FanDuel, DraftKings, and FantasyDraft (Note: FantasyDraft has two FLEX spots — allowing you to roll with as many as four running backs in a week). This thought would have registered as heresy as recently as 2015, but the development in 2016 of David Johnson and Le’Veon Bell as the ultimate Multiple backs has led to a shift in the way the NFL is viewing the running back position. For at least the next couple years (2018 and 2019), you should be looking to lock in a running back in the FLEX spot as often as you can. There will be times when salary constraints or available value dictate that a different approach is warranted on a particular roster or a particular week, but this should at least be a foundational consideration for every roster you construct.