It seems to me that a large proportion of posts in AIBU are from people who know they're not being unreasonable, but want to vent about something annoying that has happened so that everyone can commiserate with them about how unreasonable someone else has been. This leads to an irritating kind of faux naïveté from posters claiming not to know whether they have been unreasonable when no sane person could conclude that they had been. A 'vent' board would solve this issue by letting the obviously maligned rant without having to pretend it's their first day on earth.