Welllll...... I like to keep my PV in my pocket. So I take the "pen clip" off of a cheap tire gauge and put it on my 510. It's the perfect size and it works well.
With that on there, my PV won't roll off the table. And besides, when I have a manual battery on it the button stops it from rolling off the table too.
And, it's all about cost and ease of manufacturing. The thin-walled outer tube on the
batteries and atomizers is probably cheaper and easier to work with than a hex or octagon.