* Vincent Bernat <bernat@debian.org>, 2015-08-05, 12:59:
The arguments all rely on .diff.gz which is not used anymore with 3.0 formats. I think the whole entry should be removed as there is now little interest to try to enforce this rule. On the other hand, such a "rule" makes us appear as too strict for some upstreams.https://wiki.debian.org/DebianMentorsFaq#What.27s_wrong_with_upstream_shipping_a_debian.2F_directory.3F
One problem with having debian/ in upstream tarballs, is that it's tempting for upstream to put vital files there. For example, debian/ might be the only place where upstream changelog is kept, or the only place where the license is declared. Then dpkg-source would remove these vital files on unpack. :-(
-- Jakub Wilk