For example, some topics might well belong in both the Science Conference and our imaginary Mesozoic Life Conference. But some users may only frequent one of these conferences. Linking a topic between them makes the topic available to all users who visit either conference. The disadvantages to linking topics are less obvious, but very important. First and foremost is the issue of a topic's compatibility between two conferences.
Different conferences have different rules, expectations and userships.
Such links can lead, inadvertently, to misunderstandings between both the participants and hosts of the different conferences. For example, the Mesozoic Life conference is full of serious discussion, scientific fact, and proven biological premise. The TV conference is a much more social atmosphere with discussions geared towards popular culture, the production aspects of broadcasting and general banter. Someone starts a topic in Mesozoic Life on National Geographic specials that the hosts then link into the tv conference. Suddenly you have a topic full of clashing dialogue between scientific fact and discussion of content, and lighthearted banter and a discussion of Barney.