It is good to promote events.
With scenarios, folks get very emotionally attached; and we were having situations where some people would get upset over this or that aspect then go onto the boards and start a !@#$-storm.
What we found is that, when this happened, it spoiled the fun for some people who otherwise were having a blast. Many people don't like being exposed to such acrimony.
The solution was to have the ability to enforce a set of reasonable guildelines, like HTC does for its BB, like what is done in a restaurant, school, store, workplace, etc. if someone starts having a tentrum or if two people get into a screaming match. But scenario CM's cannot do that in public forums. HTC (correctly) does not give CM's the ability to moderate public forums but only the special private forums. So, the solution was to make them all private forums so that we can moderate them appropriately.
Yes, there are disadvantages to the forums being private (another one being it is an administrative pain to add people to all of the private forums). But we have thought about the advantages and disadvantages of the two ways and, based on our experiences with running a lot of scenarios, picked this way.