My forums are too "long" already without a vanity re-direct forum - if the target of the redirect is that important, it really should be linked to in the navigation

No - what about if you run a host a forum for a local study group - that is only relevant to those group members. You could include a link or two to their own community links. Those links would probably not be relevant to all your other members.
These community sub-forums are quite common - I can think of a few examples where the members of that small (or not so small) group of people would prefer to be part of a larger group, than have their own forum, which is only visited by outsiders once every blue moon. It is features like this (and the ability for mods to suspend people from individual forums), that really benefit these small groups.