Decompilers aren't terrible... they are just used to find out the entity work from maps. The brushes are too messed up to use anyway, although I could see a very determined person copying one... but still. It isn't like they are some kind of evil, evil thing, you just use them to find out how certain tricks were applied...
Anyway, BSPtwoMap can be found in the Half Life Tool Pack, which is here: http://fmc.clannfb.com/hltp.php
It helps you figure out problems and tricks without having to search everywhere for them, is all. For example, want to know how to put a model into a map? Then you decompile it, look, and then delete it.
I'll probably get yelled at lots, won't I?