On occasion, we've used WP Favorite Posts on client sites and our own ventures...mostly on sites using WordPress as a CMS. While we have used the php code with child themes, our preference is to use the plugin's shortcode (also noted in the article) which gives more flexibility on placement within a page and is not theme centric.

Since WPBeginner didn't note the client site, as a demo/reference you can check out one use of WP Favorite Posts at http://newsfeedgadget.com, our "companion" reference venture for news readers/social magazines/iGoogle gadgets. The home page (i.e., MyNFG) acts as a personal favorites page and most content pages can be favorited (i.e., Add to MyNFG).