The documentation implies to some degree that the -nc option will never touch existing files. So I tried to get new images on a site using:

wget -r -N -A "*.html" http://site.com/
wget -r -nc -A "*.jpg" http://site.com/

The second invocation reads all html files from disk and deletes them afterwords. This is surprising. A workaround could be to figure out all extensions used for html-files on the site and add them to the -A option.

I think -nc should be changed to take precedence over other options like -A. This change will probably not break any existing script, since deleting files can be had much easier.