While I prefer a plain edge 90% of the time, there is one thing that immediately jumps to my mind which serrated edges do well; keep cutting for a long, long time. Yeah, I realize every true knife knut here is jumping to their keyboards to say "just sharpen the plain edge!," but it's not that simple. Let me give real-world example.
A friend of mine is a real knife knut. Honest. He worked in a knife store for several years, sold me my first knife, talked many a long afternoon with me about the various aspects of knifedom, and encouraged me to eventually make knives myself. He can put a good edge on anything he likes and appreciates the value of a sharp blade.
This friend now works in an appliance store, where he takes apart many boxes each day. Big boxes. Thick boxes. How many refrigerator boxes can your plain-edge knife take apart before it starts to look and act like a butter knife? What if you can't whip out a sharpener midway and put the edge back on? What if at the end of a ten-hour day (and possibly dialysis that morning) you don't have the energy or the willpower to sharpen it up when you get home? What if you don't sharpen it all week, because there are bills to pay, a new house and a new wife to attend to, and a serious medical condition to worry about? How long will your plain-edge knife keep doing what it must do then?
My friend has been carrying the same serrated Spyderco Viele for over a year, and if it's been sharpened, it hasn't been often. It's halfway to being a plain-edge knife now, just due to wear, but it keeps doing its job, day after day. Not doing that job nearly as well as a shaving-sharp plain-edge, but doing it nonetheless. Real knife guy, real-world knife job.
That's the big thing serrated knives do, in my opinion.
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-Drew Gleason
Little Bear Knives