Instead of relying on the length of a body field for wether to show a read-more link or not, lets provide a more generic option:
Why don't we make an extra field from this and let actually people configure this on Field UI. The check now only works for text_summaries. That's making the assumption teasers only contain text and the full node as well. What if there's an image on there, but not on the teaser, then image_formatter_view should be patched as well .. basically any formatter in core .. and then every contrib out there should do this as well. This is a thing of the past imo.
— Swentel in http://drupal.org/node/823380#comment-6154754
This issue is a spin-off from #823380: Read More link is always visible on teaser.