There’s a key difference between math skills and tricks. A trick, usually, applies to one question, or a very small subset of questions. Sometimes the trick is extremely effective”“it might even amaze you with its efficiency when your GMAT instructor walks through it.
But what if the odds of being able to use it on the test are, say, 1 in 20? At that point, is it worth learning? I can think of dozens of tricks I know that apply to practice GMAT questions that have exactly that probability (or worse) of being applicable come test day.
By contrast, there is no underlying GMAT math skill that arises so infrequently. If you learn how to do weighted averages, it gives you an edge not only on weighted average problems, but also on conventional averages and on mixture problems. It may even come in handy on a ratio question now and then. If you really understand how to do a combined rate question, that skill will be tested in any number of guises.