I thinks it's rather a nice idea to announce departures. When people join it's hardly worth introducing yourself because who cares, really, you're an unknown at that point and no matter how honest you may be in your description of yourself, your posts will always speak more truthfully about who and what you are. Plus you can just put that stuff in your profile.
A farewell post is much more meaningful. They open the opportunity for holding memorials, virtual funerals, last wishes and tearful goodbyes. For example, if mani decided to leave he could ask his old friend Shogun to look after Ravi for him.
I think banned people should be allowed one last post, like a last meal, before being axed. It's only the civilized to do.
And matt, you are right about people rarely changing their minds after a message board debate. But it does happen sometimes. I think it happened to me twice. Anyway, it's always still interesting to know what other people are thinking and why, so you can argue more effectively in real life.