In theory, I believe it is possible to get an accurate weight one wheel at a time. In practice, the numbers are so variable that they are useless. Besides the problem that 5050 points out of the scale changing height with weight on it, there's the quality of the scales. I have never seen a bathroom scale, digital or analog, that I can't get to show varying weights depending on how I stand. Furthermore, my weight varies as much as 15 pounds from one bathroom scale to another.
I've said it before, and I'll repeat it here. I only use weights from a few magazines because I know they weigh the bikes the same way on the same scale with a full tank of gas. Maybe the weights I have listed aren't absolutely correct, or accurate, but they are consistent. And since they are consisent, they are absolutely usefull for comparison. So I don't care if your DR650 only weighs 347 pounds on your certified meat scale. For all I know, your scale is more accurate than the one at Cycle World. But I can't weigh all these bikes on your certified meat scale. And what I'm really interested in is relative weight -- how much heavier a DR650 is than a WR250R, for example. And I like to be able to look at the list and have some confidence that the difference is 66 pounds.
Often wrong, but never in doubt.