I have one and it keeps stats up while keeping the rest of the forum going fine. It's a place just for any random discussion held at the time, and that's whats so good about them, in my opinion.
Chat threads are a good addition to growing forums because it allows the members to get to know each other, however as your forum grows you will find that these threads become black holes of spam and that it inhibits growth. They also become a pain to moderate. At that stage I would create a spam forum, so the members can at least set up a new thread for each of their ideas.
Heck yes it's needed, particularly if the forum is new. I find that a lot of newbies are hesitant to start their own topics in order to discuss things, and sometimes they're afraid to spam. Rightfully so; some administrators will pound someone into the ground if they make a mistake.
Therefore, I think that a general/random chat thread is a good way to get things going.
Are you talking like a general chat thread? If you are, then it's not really necessary, but I know a lot boards that have it, so while it's not needed, it is a great way to boost up your post stats and things like that.
As everybody else has said, it's a great addition to your forum, especially if it's new. It can prove to be better than the Chatbox, and helps members communicate with each other.
On my board, I have a general chat box that runs off a thread. It's a really handy code and it works wonderfully on my board. You don't need them but it's a nice thing to have either a general chat thread or shoutbox.
Honestly, I'm at a loss as to how random chat threads become so overwhelmingly popular on certain forums. I don't find these threads addictive, but I certainly wouldn't censor them.