It's a bad concept, at least the way it's traditionally used in introductory philosophy classes. It encourages people to believe that certain patterns of argument are always wrong, even though there are many cases in which those patterns do constitute good (non-deductive) arguments. Instructors will often try to account for these cases by carving out exceptions ("argument from authority is OK if the authority is actually a recognized expert on the topic at hand"), but if you have to carve out loads of exceptions in order to get a concept to make sense, chances are you're using a crappy concept.

Ultimately, I can't find any unifying thread to "logical fallacy" other than "commonly seen bad argument", but even that isn't a very good definition because there are many commonly seen bad arguments that aren't usually considered logical fallacies (the base rate fallacy, for instance). Also, by coming up with cute names to label entire patterns of argument, and by failing to carve out enough exceptions, most expositions of "logical fallacy" end up labeling many good arguments as fallacious.

So I guess my advice would be to stop using the concept altogether, rather than trying to explicate it. If you encounter a particular instance of a "logical fallacy" that you think is a bad argument, explain why that particular argument doesn't work, rather than just saying "that's an argumentum ad populum" or something like that.