Yes it is important and my data shows in helps even with ranking (some may argue indirectly) with rankings especially when it comes to local SEO.
I am not sure about rankings, but it does greatly improve the link to your page on the SERP. We're using the following schema's for our webshop:
- Product and Offer
Combined, they show up in the results as review stars, with availability, price and sometimes even an image. A lot better than just the text, which is what we got without Schema.org.
I don't think we should overthink this Schema thing. Schema was made to help users access the most important data quickly without looking for information on the website. Therefore it is an information and not a ranking factor. But after adding Schema you will start affecting CTR for sure, which is ranking factor.
Schema is becoming important to google so that it can pick information out of big chunks of data. With SEOs now churning out 600 words length articles as a defacto standard for a good article it becomes important to find out how much of information is there in all that garbage. This information is being quickly used ways to show readymade answers on google. Eventually it may mean lesser traffic and conversions for website owners with useful data as google answers direct queries right on the SERPs. However will all that philosophy I must agree with most you here that schema mark ups are a useful thing to implement and all useful things done will positively affect your seo. You are helping SEs here and I don't see how its not improving SEO.
As a case study:
I was in 13th place in the SERPS, I noticed that #1 spot utilized schema markup for the industry, with a few schema details missing.
So I copied and improved by adding a more complete schema markup to my pages. Some pages stayed the same in the serps, the main one I was tracking dropped.
Of course it's difficult to isolate the test, but I would say I have not benefited from adding the markup.
That's something I hadn't considered.
I did not notice an increase, I don't think the schema actually brought anything extra into the snippets though, it wasn't a review or rating.