The key phrase in this question is "when you're capping".
Obviously, the most important player on the field is QB. You can't deny that.
But several people have mentioned a good point. When the backup QB is going to start, the point-spread is adjusted in order to reflect that. Plus, even with the adjustment, the public is likely to bet against the backup QB. "How can Philly win with their 3rd string QB? They're gonna get killed!"
But point-spreads are rarely adjusted in order to fully compenstate for injuries to the offensive (and defensive) lines.
I think Cleveland is a good example. Their O-line was decimated by injuries, and their offense was pathetic.