It matters if you use a consistent handle in a thread because it's simply human nature that people talk to people, not individual posts. If you're going to leave a one-off message in a thread, it doesn't matter as much. If you're going to have a conversation, it's a problem.

The membership thing prevents people swiping someone else's name and pretending to be them. If you join, no one else can use the same, exact handle. If you remain a GUEST, they can, but it's up to you to decide if that matters. Strollin' Johnny, please refrain from buggering off, whether you join or not. I like your chorus songs! (Read on, read on; another thread...)