[sfx1999]

The only real good reason to decompile something is to learn how to do stuff. It can also be good if you need to modify a map to work with updated mods, like DoD 1.0. In that case you should only recompile entities only.

It can also be used as a reference to clone a map. For example, remember pull for TFC? Someone decompiled it as a reference to make his map es_pull! for Counter-Strike. However, this is STEALING. In his case it was probably OK. He gave credit.

Also, you should definitely not decompile, modify brushed, then recompile, because of the way BSP works. It will cause a lot more polygons.