The current AdaptriveRegionGrowingTool aka RegionGrowing3DTool is not working properly and its implementation is exotic (to say at least). In addition I am not sure if its behavior is realy what most people want: some of its "magic" e.g. assumes that it will be used on CT images, its interactive adaption feature ignores connectedness (e.g. leading to islands...)...
I would argue we should just offer a new 3D tool that is closer to the 2D version. We might keept the old 3D version due to legacy. (But I personaly would deprecate and finaly remove it.)