You have to figure that motorcycles get stolen for ca$h most frequently, and for joy riding sometimes.
Theft of a unique press demo bike is a lot harder to understand than theft of a popular model for which there's always a demand for parts. (Priced plastic fairing pieces lately? Even us honest folks get a little tempted..)
In the antique motorcycle magazines, there are occasional reports of stolen rarities. That's even harder to understand. You figure motorcycles either get stolen for joy riding - and might be recovered, much the worse for wear - or to chop for parts. The CRF450R might be a joy riding candidate, but it is really baffling when something like a 1936 Rudge gets lifted - the joy rider couldn't begin to get it started, and they are so rare that if any swap meet merchant showed up with parts, they'd be immediately recognized and the merchant beaten to a well-deserved pulp.
I guess that there are folks who simply want anything they can grab that isn't theirs, whether they have any use for it or not.