I have thought of this myself and after some pondering I came to this conclusion. Adding the usergroups and permissions is just a way better way to go because someone could intentionally or obliviously share a password whereas usergroups and subsequent permissions are something that an average user couldn't circumvent.
On the other hand, I wouldn't mind having a password protection type of thing myself...in fact it would make something I have wanted to do for a while now possible as password protecting a forum is just one of those things I simply can not figure out how to do for myself. I would not be against having password protecting options for areas of the forum whatsoever.
All in all though...the permission system is quite better at handling who can see what vs a password setup and if you think about it the permissions thing is a setup once and forget about it type of thing so even if it takes a half an hour or an hour to setup the groups and permissions in the end your system is much smoother on the user end (not having to type another password after logging in.) and less able to be circumvented.
It seems llike a pretty big omission to me, some sites like RD mentioned by the top poster offer groups their own forum, so many in fact that it becomes a chore to have to set up so many groups. As a current vB site owner who has forum protection via usergroup and password its something I'd like to see before I consider migrating.
Yes like Revvin says (hi by the way ) it's a big pain in the bum to add a whole lot of usergroups just to make a forum visible. Its not that there are highly classified documents stored, people just need a place to talk quietly.
Is there another way, maybe a social groups plugin?
I think it should be a choice weather we want it to be locked down to a particular usergroup(s), password protected, or both
The reason being is if i wanted to let someone see something but i know that they don't have an account and that once they make an account to see what i want them to see and they see it they might never come back onto the forum again and then that account is taking up space. Also i would of had to go though the trouble of having to change that persons usergroup to the right permissions.
I use the feature for a Christmas puzzle for my users--they go through and solve riddles, the passwords being the answer. At the final subsection, there's a prize. So it's not an essential feature and there are definitely far better ways to lock down secret forums, but I would like to see the feature.