I've created characters who's job is to die to make a certain part of the story happen. So to give their death an impact on the other characters that's believable... I gotta make them characters that the reader will (hopefully) hate to lose. I find myself not only putting as much work into their development as I do the "main" characters, but also eventually hitting a point where I try to find ways to spare them, or create new characters to take their place... And then my inner 6 year old, the one who never outgrew pulling the wings off of flies, pulls a knife and shivs my inner care-bear... because that means the character's exactly who I need to kill to drive the remaining one's development.
That's the big difference between the typical Red Shirt and, say, one of Joss Whedon's victims, er, characters. He makes them engaging, so that the reader feels exactly why the remaining characters in the story are impacted so greatly.
But we can't just brutalize our imaginary friends forever, because people generally want to see the protagonist score some "wins." (At least in story's where it's possible. That's not always the case, John Carpenter's "The Thing" for example...) I originally loved Elric, because he was soooo unique, and appealed to my egotistical, angst ridden teen self... but eventually I got fed up with "He dies, she dies, everybody dies..." That pony can only be ridden so far.
