I celebrate and share natural milestones for the same reason that I celebrate and share birthdays--they are a way of paying attention to the way my children change and grow over time.
I really like this analogy. DD3 learned to hop on one foot the other day (yes, after much hard work), and it was such a joy to her and to the family when she finally figured it out. I have no idea whether she's early or late or average. That simply does not matter.
But no, we haven't shared it outside of the family, simply because we don't really see why others would be interested. I guess that sort of makes (part of) your point, passthepotatoes. But I nonetheless am interested in the milestones of my friends' babies (even though I've long forgotten what is "normal" timing, and don't keep track of how old the babies are anyway) because it seems like one of the only ways to communicate with/about a child who is not yet verbal. I think this sort of thing can be totally appropriate in some contexts.
