Well, to start, some people decompile maps not to modify it, but to learn how something was done in that map. half of the stuff i know about mapping isnt from tutorials but from decompiling valve maps and seeing what they did. And really, theres nothing wrong with modifying maps, its a great way for noobies to get a feel on hammer before making there first complete map...
Your right though, decompilers usually have a lot of errors. So when your run the decompiled map its a hit or miss as to whether it would work.
tcp, if you are talking about how to decompile a valve map or one made professionally, you have to use GCFscape, which cracks open GCF files so you can go into them and pull out the bsp which it is in. Heres the link:
http://nemesis.thewavelength.net/index.php?p=26
Click the installer button.
The GCF files for all the steam games you have are located in program files/steam/steamapps. Click on the game of the map you want, you'll have to look around for it, because inside the gfc files are the sounds, scripts, models and textures to the game to, not just maps.