Many, many of you seem to miss the point of character variety.
If you were going to kill Sai, might as well kill Shikamaru, Omoi, and any other somewhat silent character.
The stoic characters balance out the cast and are sometimes even more pleasurable to have about than the stereotypical, loud-mouthed, hot headed main character.
Sai balanced out Team 7 by not only being capable in close-mid range combat, but also being talented at reconnaissance, something Sakura and Naruto are not.
In terms of usefulness, his jutsus allow many, many things to become possible that weren't previously to most of the cast, such as flying. He also can be used as a plot device, because he's unusually honest compared to other characters and is true to what he believes. In terms of development, he's progressed much more than some of the older characters in the story in regards to his personality and agendas.
