1. Do you mean the BSP tree? It used to be the default tree in prior acknex versions and is well suited for (mostly) indoor or oldschool shooters and games like Doom, Quake, Forsaken/ Descent and similar.
The region_set feature is nice though, especially for suppressing stuff like content in a house when the player is outside of it.

2. The shader pre-compiler is pretty useful and should be used for any final release. You can then simply use the *.fxo instead of the *.fx files for your material effects and the shaders will not be compiled on the fly each time you run the game (which can take up to several seconds, depending on the shader complexity).

3. There are some limitations like not being able to use file_... instructions on files in the *.wrs resource but apart from that there's no reason to not use it. I find it way more "professional" too to only have one or multiple *.wrs files instead of all your game resources lying there in some data subfolder.

4. The starter definition is a joke by modern standards, it's from old A4 days but you can do some simple stuff with it (preferably hiding it I guess).