Is there an option to have all topics from all subforums below displayed in this top-level forum? I think this would be quite useful.

Surely then they might as well be in this forum

What would be the point of having subforums if all of the threads are shown in the parent???

You can have a quick overview of all topics currently discussed in the subforums below. If a user does not like to go into each subforum, he can ignore them and use the top-level forum as it was a normal forum.

Imagine you have a forum "countries" and several subforums for each country. If one is interested in a single country, he goes into the subforum. If one has got broader interest, he can easily see the discussions about all countrys in the top-level forum.