For charisma checks, don't underestimate the added options that come with having multiple specialists. Good Cop/Bad Cop, Two-Person Cons, and Working a Room, for exmple, all are only reasonable options with multiple characters competent in social skills, and all offer social-engineering outcomes unachievable with only a single face.
For a binary check I agree with you. But if a DM provides more information the higher the result of the knowledge check, letting everyone roll only ensures the group will know the basic knowledge. The results with higher DCs are the domain of the specialists, and the top DCs only if they roll well, which makes having multiple specialists valuable.