Haven't you noticed in heaps of shows she is the girl who gets broken up with then gets taken back but then ends up dumped again or similar.
Less Than Perfect - She plays Kipp's girlfriend. When Kip dumps her, he fears her and decides to take her back but then dumps her again!
Friends - When Pheobe and Mike get back together Pheobe breaks up with her for Mike.
HIMYM - She was one of the best girlfriends Ted ever had but he dumped her took her back and then dumped her again.
The reason she is always chosen for this is because she is so good at break up scenes. When she is mad/sad she is very funny because she shows it in a funny way.