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.

Quote Originally Posted by Tanarii View Post
Edit: that said, with many DMs, this also applies to Lore checks. You can get away with one lore master, and maybe an assist. But having a high Int character due to class stats, already in the part anyway, is less likely. (With DMs that allow everyone to roll a Lore check with only one success needed, you can just assume your group will pass almost all of them. For some reason many DMs don't understand they are wasting table time with these checks.)
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.