I agree, but most “related posts” plugin I have used, have always implemented a way to circumvent it. The way WP planned it to be used is clearly not user friendly and doesn’t account for many scenarios (e.g. on the Category page, off course I want full custom excerpt… but NOT if I’m displaying small intro of related posts in a sidebar or below the current post!).
Would be nice if WP Show Posts had that too.
Example the plugin I had to use since WP Show Posts couldn’t do it, is : https://wordpress.org/plugins/posts-in-sidebar/
It’s source code is available on github:
https://github.com/aldolat/posts-in-sidebar
Hope you’ll be able to implement it.
Cheers,