One word: Joyce. Total support player. Best death. However you verb the complaint, it still ends up being inaccurate; all manners of characters, primary or otherwise, got both appropriate and inappropriate deaths. Anya was a pivotal character for almost four seasons and got an ignominious death. Hell, Matt even used Doyle as an example, which is simply ridiculous considering that he was one of three primary, original characters on that show, not to mention he got one of the best death scenes. And Tara? Who decides if she's a disposable character or not, or that her death was cheap?
But he was concieved as an original, primary character with the intent of being killed and Wesley taking his place (at least according to Whedon). Therefore he was concieved to be a disposable character who created the faux-image that "anyone is vulnerable," which really wasn't the case until the final season. Outside of Doyle, how many major characters died on Angel prior to season 5? Lilah, Holland Manners, and Merl? Thats all I can pull off of the top of my head. The only major character of those three being Lilah. Lilah did not die until the end of Season 4. Certainly doesn't make me think "Hey! Anyone is vulnerable because a character they planned to kill after a short time from day one died!"
And Tara is clearly disposable. If she is not a necessary element to the show, she is disposable. The show went on without her. Thus, not necessary. Thus, disposable. And I'd say its a pretty cheap death when you keep the character side-lined for a majority of the season, bring her back for one episode, add her to the opening credits (after 3 seasons), and then kill her in such a quick manner.
What you're basically asking for is that all characters get treated ****tily, or that no characters get treated ****tily at all. Either everyone gets treated as a star, or no one does. Which is impossible, and would have made for a horrible show.
No, what we're asking for is Whedon grows some testicles and either kill a character that matters (and in turn inject some suspense into his story telling) or quit trying to create this artificially false "no one is safe" mentality, when clearly, the major five characters (Buffy, Willow, Xander, Giles, and Dawn) are all very much so death proof (and in turn save time by not developing useless side characters who die for pretty much no reason).
Oh, and just for the record, Wash, Book, Darla, Tara, Anya, and Joyce ARE side characters. They are not crucial to the movement of the plot. They are not focal points of the over-all plot arc.
In Buffy the focal points to the plot are Buffy, Xander, Giles, Willow, and later Dawn.
In Angel the focal point to the plot is pretty much just Angel. Angel and his journey alone is what drives the plot. The argument could be made for Cordeila as well, in which case I will concede, she was not the pointless elimination of a side character, but more so a character who had come full circle, completed her story, and had reached the point where from a story telling perspective, death was the best option.
As for Firefly, Mal and River are the ones who move the plot focuses on.
Basically anyone else is by definition...a side character.