Not to get bogged down into calculations and such, but the reality is the backups are getting less and less opportunities to prove themselves. They just need to be ready when called.
Last night I umpired a lopsided game that ended by run rule after 4-1/2 innings. The home coach started subbing players in the bottom of the third and had all nine of his eligible substitutes in the line up by the bottom of the fourth.
Even with a 14-run lead, he watched every at bat his subs took with keen interest. Every time one of them took a strike he didn't approve of them letting go by, he barked out a reminder that this was their chance to show they can hit and if they don't want to hit, they will sit.
One at bat in a blow out may not be much of an opportunity, but if it's the only one you get, you'd better seize it.