I think I have only ever seen 1 plugin that I have purchased come with instructions... AND THEY ALL SHOULD!!!
What I have noticed is that a good amount of developers are english second language and with that details in plugins admin panel settings are not always clear as mud... what I would love to see is instructions be a requirement of plugins so that setting them up is easy for end users.
This would be good to see.
I know some do add instructions but they usually just a cover letter on how to install, not how to set up.
I find it rare I can install a plugin and not have to contact developers for support.
Same Same!