A lot of guys will just pull one of the duals off the rear for the winter and run them that way. With single rear rims they will track different then the fronts. Can't really see any reason would affect the wheel bearings though.
With the dual rims installed, the upward force the hub places on the axle (through the bears) is more or less straight up. The weight is distributed across the surface of both sets (inner and outer) of the full-floating bearings. When you leave the outer rim off, the force is no longer straight up. This is because the remaining tire and rim is no longer centered between the bearings. Now there is a twisting force on the bearings. Since there are thousands of pounds of force (weight) being transferred through those bearings, the twisting force is significant.
Simply installing a spacer between a rim and a hub has this effect. With floating bearings which are not horizontally oriented, this probably doesn't matter too much. With full-floaters, there is extreme force on the inner and outer edges of the bearings because of the twisting force. This will wear them out in short order.
Anything you do to move the support the tires are placing on the hub will cause undue wear. The less you move the support, and the less the weight on the bearing, the less the undue wear will be. I would expect a standard single rim to be preferable to a single dual being installed on a hub intended for dual rims.
I don't know all of this from personal experience. Towcat recommended against this for anything other than short-term purposes, and the explanation I just gave is what I came up with when I mulled it over -- wondering "why?". The specific example being warned against by Towcat, to me, was not to use single-rim hubs with a spacer and dual rims -- and he mentioned someone who went through lots of bearings on a truck so-configured with a heavy camper installed on it.
I think the only way to use a single rim (without stressing the bearings) in place of dual rims is if the single tire ends up precisely centered between where the dual tires would have been. The same is true if one is mounting dual rims where there was supposed to only be one.
