While I'm very much in favor of precise measures for repeatability, ginger is a little hard to measure. You have to slice it very thin (to cut the fibers short) or grate it for most recipes, which means you're working with a whole root. I suppose you could weigh the slices as you slice it, but it's just too much hassle when it's not that critical. Volume is inaccurate, since the measure depends on how fine it's chopped.

I'm presuming cooks smart enough to be reading my recipes will be smart enough to apply appropriate windage, depending on if s/he thinks the root in hand is bigger or smaller than average. That, after all, is how I do it.

It would be a different matter if ginger varied a lot by region or variety, like daikons for instance, but it's pretty uniform worldwide.