Does a "Read More" button to open up the full content affect SEO?

Do you think it could be the case that G is still crawling & indexing such content BUT any contribution that copy may have had to the pages seo efforts is now lost if hidden. So to get its contribution to SEO back one needs to reveal it, have fully displayed ? OR no need to worry and can keep such copy behind a 'read more' button/link ?

I've read a few articles with some who advocate using it and others who discourage it. Does anyone have any history adding "Read More" to their ecommerce category page content and the effect it had?