I am glad that you are asking. I will explain how it can benefit PunBB and its users:
1. I run a small time hosting company and I right now have to use 3 different softwares in order to provide complete support to my clients i.e: Knowledge Base script, Support Ticket System and a Forum.
My client has to register at all 3 places differently in order to get help and support which I myself feel is very annoying. So if PunBB gets a private thread feature then in real I can pack all of these 3 things into 1 single PunBB.
Now you ask How ?
1. I create a forum category called Knowledgebase (Knowledge base script is taken care off)
2. Forum is already going to be there in PunBB (Forum is already done)
3. Private Threads in PunBB which will act as Support Ticket System. As this will be private, so clients will see their own threads (tickets). Why private ? Becuase clients will post their username / password etc. which nobody else should see.
This is a very basic thing which can be put in the forum rules in admin area of PunBB. phpBB and vB etc. already have this but they are slow and bulky. This will be a good additon as far as I think.
I am not sure how many people are using a forum software as a support ticket system but I think that its going to be a great reliefe for clients as they don't have to register at 3 different places, I don't have to update all the folders everytime there's a change, so its just going to be a great reliefe.
I hope many people who are using 3 different solutions may go for this. I know that this is not the best way but it works. Simply forcing the user to subscribe to the thread in the private forum and whenever admin/support staff replies the client gets an email telling that reply is there.
If I have (kikloo) has started a thread in private forum category, then its only visible by me, mods and admins. If anyone else tries to view then they get access denid or something like that.
Main thing is that with this feature added PunBB will be able to act as a Knowledge Base, Support Ticket System and Forum. All in One package! I hope you get my point.