Used to sell electronics for a couple of years and was finally able to get this explanation out of an Onkyo rep who knew his shit pretty well on the Optical vs. Coax debate:
Coax digital is the better way to go, because it is sending a verbatim signal which is in effect a direct transfer of an electrical impulse between two components, whereas the Optical signal is taking that electrical impulse and converting it into a light impulse at the output of devise A and then re-converting this back into an electical one in devise B. As a result of this, with a good piece of Coax, you will lose less detail than you will with an optical cable. In other words, it was implied that the Coax was a more "pure" and stable way of doing things.
Methinks you would have to have pretty good ears to discern much of a difference between the two though...
Not to get off on a tangent or anything, but I also heard from a couple of different people that there is a fair difference between a glass and plastic fibre Optical cable, the glass type being "quicker" and more accurate of the two and of course considerably more expensive. Same deal with the good ears I guess.
I suspect that most people use Coax for taping because it is a more durable cable, in particular the ends in the Optical format being much more sensitive to getting scratched which becomes more tricky when you are continually connecting and disconnecting that cable during field use.