Yea, it's a little shortsighted that a 2nd back's only value is blocking. The point is to neutralize a guy which could be accomplished in more ways than just blocking. You send Moss out to the flat on one side and Singletary to the other and there could be a lot of confusion. So maybe they're average blockers at best, but they have likely neutralized at least 1 guy who's covering them and possibly caused hesitation in others.