Iowa typically doesn't get a lot of backups in. Iowa typically plays a lot of close games. Last week, Iowa won by 8 touchdowns and played pretty much all the backups. Alabama, meanwhile, has won 43 games by at least 3 touchdowns under Saban. USC, from 2002 to 2009, won 53 games by 21 points or more.
Since 2002, Iowa has won 35 games by at least 3 touchdowns, with the rate dropping pretty hard over the past 5 years (with the exception of 2008, when Iowa won 6 blowouts; that represents the most such wins in any individual season under Ferentz). Put another way: Iowa has won 8 fewer games by 21 points or more in the last 11.3 seasons, than Alabama has won in the last 6.3 seasons, and 18 fewer such games as USC did in an 8-year span.
What that tells me is that Ferentz does play his backups when he gets the chance, but that chance hasn't come along nearly as often as the elite schools, especially last year (0 blowout wins).