Matthias,
I think there are a couple of issues here
#1 giving users the ability to customize their view of information
#2 flexibility in user interface of forums
Now #1 and #2 could have many components. I think one issue in forums user interface is threaded/vs flat. It does not really make sense to support switching the view per user as it would change the context of the posts in the thread.
I also can't really see an effective use of making _many_ user interfaces available across one .LRN installation. I think one user interface with a couple of options makes sense, but supporting more than one user interface package seems like it will lead to support issues that would have to be carefully considered.
Offering choice to end users is important, but it needs to be balanced, too much choice can just lead to confusion.
