I'm in favor of only opening it for cons. However, open it a couple of days before the con starts, not on the last day. That way it can function as a sort of mini-/soc/ for con-goers.
Right now /con/ is kind of nice because there's no regulars calling me a newfag, no memes I don't understand, and no need to lurk for months before I learn how to post here. It's truly a cross-board board. If /con/ turns permanent, newcomers won't be as willing to post. Which kind of defeats the purpose, I think.
Stricter moderation, disabled GETs, and a ROBOT9000 script would also improve things here, I think.