**** big fingers
Dulleys
Wonder if dual tires in back are necessary for everyday van set ups
Okay so now that I know you are asking about duallies (dual rear wheels or DRW), I can give you a few thoughts based on having a DRW E-Series van.
I would 100% say they are not necessary for "everyday van setups" (obviously depending on your setup). The way I would approach them is this: Only get them if you know you need them to meet your specs.
I've had them for quite a few years on my E-350, and while they are not a tragedy or anything, I wouldn't choose to have them if I didn't need them. Why do I have them? Rear axle weighs over 7,000#, GVW is over 10,000# and vehicle is close to 10' high and 7' wide (cutaway). There would be no practical way to make the weights work with single rear wheels.
Upsides are you can put more weight on the rear axle, and some amount of extra stability.
Downsides are you have two extra wheels and tires (and all seven are on chunkier dished rims). You may want special rigid/extended valve stems (makes it so you can easily fill with air, and takes away disadvantages of "soft" extension), but then you can't really rotate your tires in the usual way. If you are over 10,000# it may require special tow trucks and some lifts won't be able to hoist you. Rear track doesn't exactly "fit" in front track (if you are off pavement). May not have as good a traction in some situations (basically the situations where "pizza cutter" tires are advantageous - becaus you have the opposite in the rear with more surface area). Wheelwells are larger inside van. You may not have the same selection in tires. Rocks can get stuck between rear duallies.
So to reiterate: I would get them if you need them (weight, etc.). For myself, I will do what it takes to meet the criterial of a "singly" this time around. Don't hate the duallies with a passion or anything, but just would rather avoid them if doing so can still meet my criteria.