A solid telephoto zoom lens opens up wildlife, sports, and landscape shots that your kit lens just cannot reach. Good glass outlasts camera bodies, so buying a durable, weather-sealed model today means you will not need to upgrade again in three years.
Camera brands update their body lineups constantly, but lens release cycles move much slower. When a manufacturer drops a new version, last year's model drops in price quickly—and refurbished stock from the maker is usually pristine.
Keep in mind that the intro window is a deadline, not free money. If you still have a balance after 18 months, standard interest rates kick in on whatever is left, which quickly eats up your savings.