Not an expert but this is what I have come up with, researching the duel vs. super singles for rear wheel set-ups.
I would welcome other expert/layperson opinions also.
Super Singles
Much better mud and sand performance (do to rear wheels following the track of the front wheels)
Better selection for off-road tires (some limited availability because of current government contracts)
Greater ground clearance do to larger size tires in general (factory equipment vehicles are geared accordingly)
Do not pick up rocks between the tires (duel rears can minimize this with a mounted 'rock-pick')
Plus the 'cool' off-road look
Duel rear tires
Factory set-up (other then Unimog, Mann, Ural, etc) warranty and liability issues if changed?
( Darrin Fink's write-up on SRW conversions--http://www.ruf-inc.com/srt.htm )
Greater load capacity with less expensive tires (load can be match with some super singles)
Better flat / blow-out protection
Much cheaper tires / wheels for a given load range
Great selection / availably (depending on wheel size and tread pattern)
Lighter in weight is a benefit for carrying spare(s) and tire changes