In my experience, spey casting with a single hand rod is designed to make a modest cast out of a cramped environment, just something better than a roll cast.
Bingo!"For normal sized flies and room to cast I can't see much reason to do spey casts with a single hand rod."
One viable reason is instant major change of direction. Use the spey as a 'set-up' cast, then with all lined up, pick it up into single backcast and out she goes.