Regina: I know she's kind of socially retarded and weird, but she's my friend... So, just promise me you won't make fun of her! (00:01:00)
Mr. Duvall: Miss Smith, why would Regina refer to herself as a "fugly slut"?
Student: Nice wig, Janis. What's it made of?
Janis: Your mom's chest hair!
Mrs. George: Hey, you guys! Happy hour is from four to six!
Cady: Um, is there alcohol in this?
Mrs. George: Oh, God, honey, no! What kind of mother do you think I am? Why, do you want a little bit? Because if you're going to drink I'd rather you do it in the house.
Janis: Regina George... How do I begin to explain Regina George?
Emma Gerber: Regina George is flawless.
Lea Edwards: She has two Fendi purses and a silver Lexus.
Mathlete Tim Pak: I hear her hair's insured for $10,000.
Amber D'Alessio: I hear she does car commercials... In Japan.
Kristen Hadley: Her favorite movie is Varsity Blues.
Short Girl: One time she met John Stamos on a plane...
Jessica Lopez: - And he told her she was pretty.
Bethany Byrd: One time she punched me in the face... It was awesome.
Cady: You're not stupid, Karen.
Karen: No, I am actually. I'm failing almost everything!
Cady: Well there must be something you're good at.
Karen: I can stick my whole fist in my mouth! Wanna see?
Cady: No. Anything else?
Karen: Well, I'm kinda psychic. I have a fifth sense.
Cady: What do you mean?
Karen: It's like I have ESPN or something. My breasts can always tell when it's going to rain.
Cady: Really? That's amazing.
Karen: Well, they can tell when it's raining.
Cady's mum: Where's Cady?
Cady's dad: She went out.
Cady's mum: She's grounded.
Cady's dad: Are they not supposed to be let out when they're grounded?
Short Girl: Hey, get out of here.
Damian: Oh my God - Danny DeVito! I love your work!
Gretchen Weiners: Maybe she feels weird around me because I'm the only one that knows about her nose job. Oh my God, pretend you didn't hear that!
Coach Carr: At your age, you're going to have a lot of urges. You're going to want to take off your clothes, and touch each other. But if you do touch each other, you *will* get chlamydia. And die.
Cady: I know it may look like I was being like a bitch, but that's only because I was acting like a bitch.
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.