Well, there are more issues than that. For one, if you're able to log in as me, you're able to see all the private messages that I ever received or sent, which could be pretty damn uncomfortable for someone if there was anything confidential in there (there isn't because I just checked and deleted everything of the sort). You're also able to gain access to all the forums that I can access, which is not particularly impressive because I don't have any special privileges to see any more forums than any other regular user, but certain people (e.g. armchair treasure hunters) do have such privileges and if confidential things are being discussed on those forums, that could be very serious as well. You're also able to see my ignore list, which is definitely going to be at least a little uncomfortable for me, although nothing too serious. You're also able to see my forum settings, which you're not really supposed to see, but I can't imagine a plausible scenario where this causes any problems even if there's a breach.
The shared passwords concern is also very real.
Of course, none of this is a problem if people act responsibly on the Internet, but it's still important to maintain security at multiple layers because something is always going to go wrong, especially when people acting responsibly is supposed to be a part of the plan.