[mild rant mode]
Sure it would be awesome to work only with Debian proper, but due to
dfsg and probably other legal concerns several packages will never be
part of the distribution, or only limited versions. d-m is pretty much
playing the same role that rpm-fusion is playing for Fedora or
medibuntu
for ubuntu, and in both cases those external repositories are
essential
to work with multimedia when you can't have total control over the
files
you are working on. Sure you can do without, and recompile from
sources
what you need with the proper flags/codecs, but what good it is
compared
to getting it from an apt repository, where versions are upgraded
regularly ?[/mild rant mode]