I didn't mind Kendra, myself.
I believe that she was neccessary, in a way, in the show, whether the writers thought of it this way or not. We're told that Buffy is an unconventional Slayer, and how this proves an asset to her on many occasions. Then in Season 3, you have Faith, who certainly doesn't like to rely on anyone else, but is, in her own way, another unconventional Slayer- she's not obedient, she's emotional, etc. Apart from Kendra, these are the only two Slayers you see until Season 7. Kendra's very much an example of the "model Slayer". She studies, she's knowledgable, she's socially inept, she's obedient, she lives for her work as the Slayer. She changes slightly during her time in Sunnydale, but I think really, that's the main purpose of her character.
That, and notching up a Slayer death to Drusilla 😛