svn:ignore version controlled resource

I understand the svn:ignore property currently doesn't apply to
resources already under version control, but our project is in this
common situation: http://subversion.tigris.org/faq.html#ignore-commit

We've worked around it, as suggested, by creating a "databases.yml.tmpl"
file and removing "databases.yml" from version control, but I can't
figure out for what reason the svn:ignore property shouldn't apply to
version controlled resources?

Thanks, Jack