I find this very inconsiderate. For most articles, intro image = full image. Why do users have to do the "assign image" chore twice? Why can't there be a setting to display intro image in the single article page?
I don't agree. I never use the same image as intro- and full-text image. If you want to do that, it's very simple to copy/paste from intro- to full-text image, when have picked the into.
And if you really wan't not to do that, you can make a very simple override in /html/layouts/joomla/content/intro_image.php - just change image_ with image_fulltext
Intro-Image can be a Part of Full-Image: as there is a smaller Size for Intro-Image in for Example a Blog-View i prefer to show a Part of used Full-Image.