I think the idea is that for most teams, the backup to the starter probably isn't going to step into the role and be a stud if the starter goes down. Obviously exclusions would be Pollard, Mattison and Dillon (and probably a couple more), but if you're backing up Henry, CMC, Ekeler, Taylor, etc, you're spending a roster spot on someone who probably isn't even going to get the role fully to themselves and is going to be considerably worse than the starter was, in the event that the starter even gets hurt. There are better bets to make with your roster spots. I'd rather target opponent's backups and try to get lucky with those.