It takes one five-minute session every few months to clear out the old subscriptions. They're all sorted by last post date, so it's easy to jump to the last page, select all, delete, until the number is down to the non-scary level again.
You don't, but I don't like the clutter. I also unsubscribe from threads I posted in earlier that have since gone off into the usual uselessness, who wants to see new posts in those?
I guess since I only look at the "New Subscribed Threads" list I don't see the clutter, since it only displays the newest 12 or so threads (in order by the time of the most recent post).