The mailing list can sort things much better than a forum, and bugzilla does even better.
You can browse them at least as good as a forum, they also prevent duplication in a better way, by using the evergreen search function
Moreover they force you to organize the way in which you expose problems before posting, in a forum you could be tempted to write quickly and post without paying much attention.
I also agree with sabayonino
, having mailing list, bugzilla and a forum section is useless duplication of efforts in my opinion.