hugocentrique – Most users want to track several different things on their site from lead capture, to views of important pages, to how many people added a product to their cart, to how many people converted/purchased something, etc. This is precisely why when you go to create a new pixel, Facebook prompts you to select one of the following categories: Checkouts, Registrations, Leads, Key Page Views, Adds to Cart, Other Website Conversions. Choosing to use only one pixel that will execute on your entire site on every page load will prevent you from being able to tell how many users opted in to your newsletter vs. converted/purchased, vs. whatever else since the same pixel will be executing 100% of the time. Watch this short video for more info.
One exception to this would be choosing to set up ‘People visting specific pages’ rules in the Audience builder interface (this is described on the Facebook Docs page you referenced). However, that involves manually typing in the URLs of every page you want to track, as well as choosing logic statements such as contains/does not contain/equal to/not equal to in order to try to account for every possible wildcard/variable URL your users may land on. As you can imagine, that method is not very user friendly for site owners, and is prone to induce mistakes. The preferred method for most users is to set up separate tracking pixels for each individual conversion point they want to track, then insert those individual pixels into only the corresponding pages of their site using this plugin.
I hope that helps to explain why this plugin allows users to insert conversion pixel codes on individual posts/pages rather than inserting only one for their entire site. Please consider re-rating this plugin with 5 stars, since it does exactly what it says it does; nowhere on the plugin Description or FAQ pages does it state that it allows users to insert conversion pixel codes across their entire site, yet that’s the reason you cited for giving it a 2-star rating.
Thanks for your interest in using my plugin, and please let me know if you run into any other issues.