what problems is the open internet thing gonna cause?
You know how stupid people sometimes get when they're arguing about something unimportant on the internet, and then they start attacking the person in every way they can? Digging up old threads to annoy them? Finding their email and subscribing them to peta alerts? Finding them on facebook, or blog, or elsewhere and annoying them?
Imagine if they knew your address. It's very easy to fill out a magazine subscription with "bill me later" checked. But there are lots of other worse ways to annoy someone you feel has done you a grave wrong.
Further, for those of us with children, we can either post pictures of our children online, or post our address, but it's not particularly safe to do both, because there are some sick people in this world, and even though it's only a 1 in a million chance (and it's probably lower than that anyway) as a parent can I justify taking that tiny risk when it can so easily be avoided?
Lastly, somewhat like hanging out in a private clubhouse, some want to come here and unwind in ways that might seem at odds with their life. They don't encourage their friends to visit them here, and they don't encourage people in the forum to get to know them in real life. It's not necessarily that they are hiding anything, but compartmentalizing things gives them a sense of freedom they might not otherwise have.
There's also a sense of control. You can "turn off" your online friends merely by disconnecting. The sense of control over your relationships when you know they can't bug you unless you choose to be bugged is liberating for some.
Those are a few reasons, certainly not a comprehensive list.