I recently played with a plugin which I liked a lot, but felt it was missing something. So as first stage I went and wrote a tiny plugin (my first), that just added the needed feature - meaning, that the original plugin still had to be installed for something to work.
Now I gave credit to the authors for doing most of the job, but I have to think of the future, what happens if they decided to stop developing that plugin or don't resolve issues regarding it. My plugin will stop working or will potentially become incompatible with future Wordpress versions.
Would it be ethical for me to just avoid the issue alltogether by copying their code into mine and renaming the plugin functions? This of course causes other problems, but now they are my problem to resolve. What say you?