The thing about running back by committee is that even if one or two guys in your committee go down, you can find a more than adequate replacement on the free agent heap, even in the middle of the season. Case in point, BJGE last year...Cedric Benson was a midseason pickup for the Bengals last year and was decent enough.
This is true; check out Adam Caplan's list of best avail. FA RBs:
Scout.com: Updated Best Available Free Agents-Offense
On the other hand, if Seymour or Warren gets hurt, you have Mike Wright, but beyond that zero adequate replacements will be available in free agency in the middle of the season...
Also prob. true; Vonnie Holliday & Anthony Weaver seem the best avail. FA 3-4 DEs:
Scout.com: Updated Best Available Free Agents-Defense
Position scarcity isn't just a buzzword for draft strategy, people.