Regina: Let me tell you something about Janis Ian. We were best friends in middle school. I know, right? It's so embarrassing. I don't even...whatever. So then in eighth grade, I started going out with my first boyfriend Kyle, who was totally gorgeous but then he moved to Indiana, and Janis was like, weirdly jealous of him. Like, if I would blow her off to hang out with Kyle, she'd be like, "Why didn't you call me back?" And I'd be like, "Why are you so obsessed with me?" So then for my birthday party, which was an all-girls pool party, I was like, "Janis, I can't invite you, because I think you're a lesbian." I mean I couldn't have a lesbian at my party. There were gonna be girls there in their bathing suits! I mean, right? She was a lesbian! So then her mom called my mom and started yelling at her, it was so retarded. And then she dropped out of school because no one would talk to her, and she came back in the fall for high school, all of her hair was cut off and she was totally weird, and now I guess she's on crack.
Cady: Regina said she'll talk to Aaron. And now she is. How can Janis hate her? She's such a good... SLUT!
Gretchen: Growing up female in this world is not easy. In China, baby girls are routinely put up for adoption. And in parts of Africa, women are still made to live in tents during the time of their menses.
Karen: Ew!
Gretchen: And even in fancy countries like the United States and England, seven out of ten girls have a negative body image.
Regina George: Who cares? Six of those girls are right!
Damian: She's fabulous, but she's evil.
Cady: Half the people in this room are mad at me, and the other half only like me because they think I pushed somebody in front a bus, so that's not good.
Regina: I know, right?
Gretchen: I'm sorry that people are so jealous of me... But I can't help it that I'm popular.
Mr. Duvall: So, uh... How was your summer?
Ms. Norbury: I got divorced.
Mr. Duvall: Oh. My carpal tunnel came back.
Ms. Norbury: I win.
Janis: There are two kinds of evil people in this world. Those who do evil stuff and those who see evil stuff being done and don't try to stop it.
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.