Is variable temperature control worth paying for?

For tea drinkers and pour-over coffee, yes. Reviewers consistently note that green tea, oolong, and coffee each want different water temperatures, and a kettle that hits and holds a target (the Cuisinart PerfecTemp, Cosori, and Cocinare all get praise here) removes guesswork. If you only make black tea or instant coffee, a simple boil-only kettle saves money.