Passing the light cord under your long hose is a problem? Light in the left hand and donate with the right.You would also need to deal with the cord in the event of donating one's primary to an OOG diver. It's a rare event, sure, but you don't want the cord to get in the way, so you need to do the drills dealing with the cord. Those are a PITA. I suppose if I were a more experienced diver diving more frequently it would be a non-issue. I'll likely remain a "tourist cave" diver and switch to a cordless light.
Nothing like donating with the light shining right in the OOA diver's face.