I see two issues:
First, a lot of chatbots learn by being fed conversation. On a game, especially a single player one, it may not get the necessarily interaction. Might help to pre-load it as part of building the game.
Second, you've got to firewall the way the chatbot learns, or it might learn faulty data that results in it not playing the right role in the game. Imagine a game where the "conversation" the robot is fed includes speculation on a murder. The robot decides that "The killer is Detective Martinez" is a good sentence to spurt out, but it was supposed to be the character that reveals it's actually Admiral Hooblescooble.
Most chatbots are focused around language structure. They produce syntactically valid sentences, but the best they can do for meaning is correlation-related usually. "Cherry" corresponds with "MX" more than it corresponds with "membrane", for example. This limits how useful they can be for anything but smalltalk.
OTOH, they can be great fun for smalltalk-- if you find an IRC channel that allows them, for example, they'll learn quite entertaining foul mouths.
