Simple user comments are a very important part of the internet in recent years, as they are found almost everywhere, such as blogs, video platforms, social networks, etc. Their usefulness opinion is different, but it's Google Believe they could find the way to the search engine Google Search.
As the 9to5Google website says, the company is proven with some users, allowing them to comment on the search results for sporting events (currently). The annotation system looks like this YouTube with other users able to give a positive or negative vote in your comments.
We do not know whether adding an annotation system will help the search engine, and it's probably that's why it's just in a specific category. As well as the quality of these comments, a factor of concern is the possibility of a more confusing and "worried" use environment.