I agree with everyone here. The question was, has anyone found a better way to delete the unused files. They haven't. That said, an enhancement request is the way forward. Hopefully if enough of us do the same as Angelo, Adobe will take action :-)
For removing unused images, see: wvanweelden.eu/product/image-cleanup (free script)
What other file types would you like to delete? It would be pretty siple to create a script to remove all unused files (that is: files that are not part of the RoboHelp project.) Of course, if a topic is part of the project, but not used in TOC/IDX/other topics, etc. it is still used.
Greet,
Willam