I felt the connection between Tara and Willow, and I was sad to see her go. I read things written, that it was a cheap shot, that they killed tara for willow to go evil for two eps. Tara's Death had been planned long before it happened. They had planned for willow to go evil in five, they had planned to kill Tara, in five. So actually I think we got to hang on to Tara and see her longer than we were originally going to.
I have also read that Joss, was really fond of Amber Benson, and the character she made her own, in Tara. He said in and Interview of all the Deaths Characters had, Tara's was by far the hardest. It was incredibly hard for Amber too. Everything that happened on the show was planned up until seven to my knowlege. So I don't think Tara's Death was decided in haste.
however correct me if I'm wrong