for the writing boards i agree-- leave old threads alone. makes sense.
for the discussion boards however, i'd prefer more, not less, flexibility on this concern. some issues stay relevant, or "hot" for extended time periods. yes, to revive discussion on a given topic, someone can simply start a new thread. but kicking an older thread up instead of starting over affords a chance, at the participant's option, to read through previous thoughts on the topic before adding their own thoughts.
i'm surprised to hear that newbs kick old threads up (with insubstantial posts) just to get around the 5-post rule. that seems kind of strange. is it really that hard to get 5 posts in? just introduce yourself, tell what music you're listening to, and who your favorite authors are, and you're more than halfway home.
anyway, just a coupla thoughts ...