Well, NPC party members aside. There is nothing better than a real player Sure the Companions and battle Chocobo's can fill in the gaps so the party can keep fighting the good fight. I'd still wouldn't mind spending the few seconds to summon a real player to the party vs, whipping out my NPC Chocobo.

1 other thing I forgot to add. When your adding the last player to the 6 player party. or have a full party already but want to prep someone to replace somebody that is leaving. You can use the summoning stones to summon a player despite having a full party. Then when your ready you can add the player to the party when the other player leaves.