Previews will continue the carnel tradition of being a precursor to a potential review. A review has to be based on actually playing something as that is the function of an RPG, simply reading a product is not enough. A preview though might provide useful information on the nature and structure of a product.

In terms of spoilers both previews and reviews are aimed at the potential audience for a product and for things like scenarios the assumed audience is a potential GM who is going to know the plot in advance anyway. Games are not narratives.