When running this:

$ wp media regenerate --yes

The command runs on ALL attachments, regardless of file type.

This results in lots of awkward errors when run on audio, video, PDFs, etc. Here's an example:

...
Warning: 23/23 Couldn't regenerate thumbnails for "2012 Guest gallery proposal guidelines" (ID 20).
Error: Only regenerated 16 of 23 images.

In the above example, the media library contains 16 images and 7 non-image files.

It would seem wp regenerate media is naturally intended only for images and so it would make sense to only run it on a whitelist of filetypes. This would also avoid issues like this one.