"When a show closes it’s always noted it played x performances with x previews, so obviously the industry doesn’t count them (nor does IBDB). But to the actors and the audience they’re a performance like any other. One of the great inconsistencies."
Again, this dates from back in the day, when Broadway shows had VERY few previews, AND the ticket prices for preview performances were significantly lower. As the years went by, "preview" periods began to get longer and longer, and the ticket discounts disappeared. As a result, there gradually came to be less and less difference -- some would say, no difference -- between previews and regular performances.
That said, the difference in total number of performances, whether or not previews are included, is still relatively small in most cases. But of course, there are major exceptions -- like (shudder!) SPIDER MAN.