IMO, the BNVD feature of being able to rotate one or both tubes out of the way of the eye is useful for guys who have a dismounted patrolling capability as part of their mission. The ability to blend the aided and unaided views of an environment, especially in areas with alot of ambient lighting, is valuable. The BNVD allows you to have all of the advantages of both binocular and single tube NVGs in one package, and rapidly transition between the two as the situation dictates.
Having said that, fixed tube binocular NVGs can be adjusted high enough on the mount to allow you to see under them, giving you some of the same advantage, minus the Binden effects.
In my experience, it usually comes down to cost; how often do you need the capability vs the size of your budget.