Many Web developers and SEO experts are interested in schema.org markup because they want to trigger Google Rich Snippets, i.e. the augmentation of search results by stars for ratings, price and inventory level information and more. This is a valid interest, and while we still lack quantitative evidence, it is pretty obvious that additional information (like the price of a product), in particular positive signals (five-star ratings or availability on stock) can increase the likelihood of a click.
Note that this will have multiple effects for search engine performance of a site:
First of all, it will lead to more traffic to your site, if people are more likely to click on on a result with Rich Snippet features than on others. This could either strengthen the appeal of the already prominent results on rank 1 – 3, or as well bring back into the game the otherwise unfortunate lower ranks in the results. For instance, if a result on rank 7 is the only one with Rich Snippet features, that one could become more attractive for search engine users than a result on that rank without any special appearance.
Second, Rich Snippets could reenforce the already good reputation of high-ranked sites. If Rich Snippets make visitors find faster what they need and direct them to the best match, all signals that measure user satisfaction with Google’s ranking will send positive feedback to the ranking ecosystem.
Third, Rich Snippets can send more qualified traffic to your site, because search engine users make a more informed decision on whether to visit your site.
So far, things are simple and positive. If you add schema.org markup properly and if your remaining SEO strategies are white-hat and fair, chances are high that you get Rich Snippets for a substantial subset of your pages.
But what will happen in the future? Well, be prepared that Rich Snippets will turn from something that you “control” to something that will be highly dynamic. Do not take it for granted that, in the future, Google will show Rich Snippets for all of your deep links to all visitors and queries alike, and that Google would activate all Rich Snippet features (e.g. price, inventory level, ratings, reviews, …) in the same manner.
Instead, I expect that the more structured data we find in Web sites, the more will Google selectively use Rich Snippets and future similar features to highlight small, significant details in search results. Same as with a text-marker, you do not highlight every single word even in the best of books, but only those words or paragraphs that matter the most. There is simply no gain for the user in showing fully-fledged Rich Snippets for ten of ten relevant search results.
So what will Google do? Here is my opinion:
First, they will scientifically monitor the ideal (from a user’s perspective) number of results with Rich Snippets (I guess it will be between three and four of ten items in the results).
Second, they will try to understand which pieces of information will benefit most from highlighting with Rich Snippets. For instance, if only two of ten sites indicate availability of an item, that will be more useful to show in a Rich Snippet as compared to a list of results with all or none showing positive inventory.
Third, I suspect that Google will use Rich Snippets to highlight diversity in the search results. Even as of today, it is a known topic in recommender systems research that the controlled inclusion of variety (e.g. a few less matching items) can improve user satisfaction. A typical example is a query where the search engine is uncertain about the exact meaning of the query (the “query intent”). So we can assume that Google includes a bit of less relevant, slightly off-topic results in a search even as of now, and Rich Snippets could be used to highlight this variety. Take, for example, in a list of products the only one with a price grossly higher than the others (e.g. the only professional camera in a list of amateur cameras). Google could turn on price information just for this one to point you to this more expensive item on rank 7.
I think we should understand that Rich Snippets and structured data markup are just one of many means for improving the human user experience in search, and I bet Google will use Rich Snippets and future visual features very cleverly for improving the interface between human minds and information on the Web.
This does not mean that schema.org or Rich Snippets are overrated. Quite to the contrary, they fundamentally transform how companies communicate with target audiences about their products and services. I just want you to be prepared that an effective online strategy for using structured markup does not end at asking a junior Web developer “to add a few schema.org elements in Microdata syntax.”. The whole infrastructure of digital marketing is changing.