I don't know the grandparent's reasoning, but for me, having a small 0 there means I loose the peripheral vision effect of the icon. That little glowing "0" is hard to distinguish from another number in quick glance. If there were no icon then I could quickly determine the most important piece of information to me - do I have mail or not?
Screenshot of the "0" and "2" from two Gmail tabs I have open side by side:
I tend to keep my inbox fully read. It's my staging area - Unread = totally unprocessed, Read = TODO, Starred = Deferred TODO or important later. For me, a quick glance for unread (any amount) is more useful than exactly how many.
