I’d say it adds gastronomically quite a lot to the dish and would say its sparsely used by places because they don’t want to spend much on fresh basil. I’ve been to Naples, several times, and have yet to encounter a Pizza Margherita that lacks basil (the plain cheese pizzas were usually called something like “pizza al formaggio”).

If you knew the history of the Pizza Margherita, you’d know the basil is an important aspect, particularly to the guy who invented it