Actually, this is very simple.
There's, 127,076,183 Japanese, as of 3/09. Let's eliminate the preverbal, the senile, the developmentally disabled, the autistic, and the deaf mute. I've no idea what we've got left, but let's say it's a nice round number - 100,000,000.
If you go up to any one of that 100,000,000 people and said, "Excuse me, where is the nearest Shinto shrine?" They'd point to it. If you asked, "Do you practice Shinto?" Most would laugh and say, "Well, I go to the shrine to ask for" a better business year, or pass a test or get married. Whatever.
If you went up to anyone of those people and said, "Where's the nearest Shintao shrine, they'd look at you and go, "Ehh? Nani?" They would not understand. NOT one.
You think I'm over-exagerrating? I'm not. I lived there too long.
If you make the slightest mistake in pronounciation, people do not understand. I remember almost crying in frustration with the following. I was on a bus and wanted to get off at Oukeikubo, a stop on the line. I asked the bus driver to tell me when we arrived at Okeikubo. (See, I didn't add the almost silent u). he didn't understand. What else could it be? The stop was on the bus line. I thought maybe he was messing with me. But no one else on the bus understood either.
I just gave up - sat in my seat and eventually the automatic announcement went off, Oukeikubo. I stormed up and said, in Japanese - colloquial fluent Japanese, BTW - "There. That's what I said!" And he looked at me, honestly bewildered and said, "Oh, Oukeikubo. Why didn't you say so."
In a language comprised of a syllabary of only 55 sounds, every detail matters.
So it's not arbitrary at all.