Not to rain on anyone's parade here, but I think killing off Jenny was a pretty good idea. Of all the regular characters, she contributed the least to the real storyline. Granted, she spent a lot of time flirting with Giles, but I don't remember her contributing a tremendous bit to the real action of the show until it was revealed that she was really a Gypsy spy sent to keep tabs on Angel. I mean, really, just what the heck is a "cyber pagan" anyhow?
If there were going to be someone else chosen to be killed, though, how about Hank Summers? They could've done a couple of episodes where Hank comes back, says he's learned his lesson, had his middle aged fling, and beg Joyce and Buffy to forgive him and take him back. Things would be beginning to look really great for Buffy as her parents begin to reconcile and to slowly put the bad times behind them. Everybody is thrilled for Buffy, there are hints o an outdoor wedding in the park followed by a romantic honeymoon cruise in the Caribbean, then...BANG! Angel does his slurpy thing (maybe even turns Hank so that Buffy has to stake him to save Joyce) and we're smack dab in the center of Devastation Central, with Jenny standing by on the sidelines still flirting with Giles while Buffy sweeps Hank up in a dust pan....And the show goes on.