KTMax,
I've had handguards on 2 of my previous bikes, the last one being the Buell as standard. I prefer to have them as on longer journeys they do help keep my hands warmer and drier. For me the mains reasons to have them would be to be able to wear summer gloves for more of the year for better feel and control, lever protection and (on my old bikes) the looks too.
However, I have not seen any handguards on an SD that improved it's already great looks yet. Therefore I've not yet invested.
Actually I think the Buell handguards have the most SD-like design of any out there, a nice black angular shape, with a strong profile. Trouble is fitting them could be tricky, but if someone managed it I reckon they'd be great !!