One reason serrated edges have been popular in recent years is that, if you can't or for whatever reason won't maintain the edge, a serrated edge will remain marginally effective after a plain edge has given up. In other words, if you work in a warehouse and cut cardboard all day and can't take the time to sharpen your knife, then you might want a serrated edge, since the teeth will allow you to saw through the boxes even after the edge has dulled. Assuming you can keep it suitably sharp, though, a plain edge might actually work more efficiently in this situation.
The real advantage to serrations, as far as I'm concerned, is that with some materials such as plastic ties, wires, etc., the teeth grab hold and keep the material on the edge long enough to complete the cut, whereas a polished plain edge will just slide off without cutting. The drawback to serrations is that they tend to snag on loosely wrapped ropes and cords, cloth, and that sort of thing. The solution, as Joe Talmadge has pointed out, is to not polish the plain edge, but rather sharpen it on a coarse stone. That way you can get a useful degree of "grabbiness" without the snagging effect.
I carry both a plain edge and a serrated edge every day, on two separate knives (I detest combination edges). The plain edge gets a lot more use, and if I had to choose just one or the other, I'd definitely take the plain edge. Bear in mind, though, that I can and will sharpen my plain edge on a daily basis.
I hope this helps.
David Rock
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AKTI Member # A000846
"Never carry a knife shorter than your schnoz."
[This message has been edited by David Rock (edited 21 September 1999).]