Whether or not Charisma is a dump stat, or how "dumpy" it is, depends, here is why:
The reaction modifier portion of Charisma is always* useful (*As useful as your DM chooses to make it that is).
How big is your game group, how many players will there be? What game system are you switching from? How many characters will there be in the party?
The reason I ask is that these retro-clone games have a fundamental difference when it comes to making characters. In the retro clone games it is more about generating characters as opposed to creating characters in more recent RPG's. According to the rules abilities are rolled 3d6 in order, so the dice fall where they may. RIght off the bat I think this essentially kills the whole concept of a dump stat since players don't get to choose which rolls go where.
I currently have 2 game groups with 4 players in each. For a number of reasons I decided to have each player generate and play 2 characters. This means that each group has a party of 8 characters, each party has plenty of variety in classes and races and have not had to hire and retainers or other aid. In this case the retainer morale and maximum number of retainers that can be hired portion of Charisma is essentially useless which means you could say that Charisma is a dump stat.
My game setup is not typical though. Typically each player plays one character. In this case each of my parties would have 4 characters, randomly generated with 3d6 in order rolls (I also make them roll for HP at level 1. Harsh? I don't think so

), and there is a good chance they would be missing a cleric, magic-user, or thief. In this case Charisma is not a dump stat as it will be important in determining how many retainers a character can have working for them and just how loyal those retainers will be.