In general, serrated edges tend to sever materials more aggressively than plain edges. They cut fibrous and semi rigid materials like plastic sheet better than plain edges. In some cases, they sever materials sharp plain edges cannot cut at all. And they do it many times longer without sharpening. Good serrated edges like those of Spyderco, Microtech, Mission, Benchmade, etc. are easily sharpened with a Sharpmaker.
But in my experience, serrated edges are mainly for severing. Plain edges are much more versatile. If you want to make things out of wood, you need a plain edge. It is extremely difficult to use serrated edges for fine detail work. And serrated edges cannot shave or scrape efficiently. Sharp plain edges sever most materials as well as plain edges but require more frequent sharpening.
Many emergency workers carry serrated edges because they are more likely to work when called upon (like severing seat belts). This is because the need for sharpening is so infrequent compared to plain edges.
My own preference is for plain edges. Like most of us on Bladeforums, I can easily keep them sharp all the time and I like the versatility.