It's not "worth" doing them, as half an hour of questing in Outland will get you better gear. But people still do them from time to time for fun, or rep. So keep an eye on the calendar and guild chat.
And if anyone is doing old instances, and goes from, say Naxx to Sunwell to AQ, it'd be nice if you'd mention in guild when you're moving on to the lower level raids, if you've still got space. Someone who's level 65 isn't going to sign up for a Naxx raid.