By reading the documentation I thought that clean would honor ignore-glob.

Excerpt from help:
Delete all "extra" files in the source tree.  "Extra" files are
files that are not officially part of the checkout.  See also
the "extra" command.

For me this implies that clean deletes everything that is shown in extras (by the way, there is a typo, the command is extras :). For me the natural thing would be to honor ignore-glob by default, as git does. Otherwise, an option to do so would be really useful.