Continuity mistake: When Cady is at the Halloween party her face is painted white/gray. when she comes through the door to see Damian and Janis her face has changed from white/gray to a more green.
Continuity mistake: In the wings after the Jingle Bell Rock dance, Aaron has his arms around Cady and Regina. Just as Kevin leaves we can see Aaron's hand is on Cady's shoulder, but in the next shot it is down by his side.
Continuity mistake: In the scene near the end of the movie, where Janis is dancing with K.G., after Janis tells him that she's Lebanese, she smiles. The next shot, she's not smiling.
Continuity mistake: When the Regina posts the Burn Book around school the "wanna-bees" are fighting. One girl goes, "You bitch!" and the other goes "You're the bitch!" and they start to fight. While they are fighting they jump onto a table and it breaks. If you look into the backround, you see Jason and his 2 friends turn around because they heard the noise of the table breaking and the girls fighting, and Jason's friend on the right puts his hand on Jason. When the camera changes shots his hand is on his heart.
Continuity mistake: When Regina is by her car after Cady's party, Regina says something about the Caltine bars and her boyfriend says that they are for weight gain. Regina spits it out and at one point you can see her boyfriend putting his cup up to his lips, then the shot changes to the other side of the car and his arms are down straight.
Continuity mistake: In the 'Jingle Bell Rock' scene, right after Gretchen kicks the stereo off the stage, you see her feet, and they are both symmetrical next to each other, then when you see her again one of her feet is behind the other. Gretchen does not move at all between shots.
Continuity mistake: When Regina is talking to Cady about Janis, Cady is wearing a light blue ribbon in her hair. As the scenes cut in and out of Cady the ribbon changes from front to back a few times.
Answer: Regina enjoys manipulating people, and she views Cady as a relatively easy person to manipulate, finding her more useful/amusing to keep in their circle. It's a running theme of the film that the friendships between all of the Plastics is very tenuous, because of the catty, manipulative nature of the girls.
Purple_Girl
Also, if Regina tried to ban Cady, people might think she was threatened by Cady. Better to keep Cady around and attack her in subtle, manipulative ways.