Just came across a great reason for using Flash decompilers, like the Sothink SWF Decompiler. I've found that Flash CS4 is quite unstable, and when Flash crashes, loosing even 20min of work is a problem. As long as you keep compiling the swf, you can get back some of the your lost work.
Unfortunately, if you have created assets that were not included on the stage, they don't make their way into swf.

Aside from that it's great if you're working on someone's website that has swfs that need to be updated without access to the source files, which happens way too often.