Actually, to be very blunt: CCing people is absolutely the only way to deal
with massive ammounts of email and very-high-traffic lists when you *care*
about not ignoring email that you should have read.
If you want an example of a CC policy radically different from Debian's,
take a look at the development mailinglists for the Linux kernel and all
related projects. There, the policy is that you are to *always* CC everyone
that should (or might even remotely need to) get an email, in addition to
sending it to the lists. Otherwise, the chances that such an email will be
lost in the ocean of stuff, or never reach the right people.
In the end, it boils down to the fact that most people have lame mail
filtering setups that cannot sort delivery mailboxes in the right priority
and do proper destination-based duplicate supression (so that you can get
automated "if it is also destined to a Debian ML, file into the ML folder,
and have any further duplicates supressed), and are not in any hurry to
deploy one.