http://www.simviation.com/cgi-bin/yabb/YaBB.cgi?board=airc;action=display;num=10...Read the "Converting planes to FSDS" topic, the same thing applies.
You can't decompile post-2002 SDK models. When you decompile pre-2002SDK models (i.e. built for FS2000 and earlier) you won't get animated and textured parts, and, given the way the models were built, you'll probably rebuild most of the parts.
In short, it's easier to build from scratch.
Not to mention the fact that most of the original builders would not appreciate someone decompiling their models.