^Well, the theories are going to have spoilers from the latest episode regardless. Also, there are regional differences upon when the episodes air. With no spoiler tags, it could be bothersome for those who aren't yet able to see the episode while somebody here from, say, the USA is already talking about it.
However, despite all this, I personally don't think Spoilers should ever be accompanied in a LOST thread for the very reason that everything discussed will be a spoiler and it's somewhat common sense not to read the thread if one isn't caught up (especially if not caught up to the Season described in the thread title). Last time we had this thread (before WiiChat was down) I think a moderator reminded everyone to honor spoilers in here anyhow, so I just assumed it was out of us regular members' hands at this point.