While I do agree that substrate can be a matter of aesthetics, it was the only choice hobbyists had for a long time. Now, we have better choices and much more knowledge of what is best for our microenvironments.
DSB's are the newest technology, based on the new understanding of what makes the oceans "tick" so to speak. It is the most natural, not only to the animals we house, but the most natural filter that money can buy. It is almost completely maintanence free (allowing that you have seeded it correctly) and pleasing to look at, as it replicates a natural reef look. (Not all reefs are made of complete rubble = CC.)
With all these positive points and no negatives (except for perhaps initial cost), why choose anything else?
I have been in this hobby a loooooooong time. I have had corner filters and CC substrates (even had crushed oyster once). I remember the days we argued weather the floss should come before or after the charcoal (boulders as big as coal)
But I have never seen the strides this hobby has made so quickly in the past 5 years or so. I have never had so much success in keeping corals and anemones as I do now. And I can tell you I have never spent less time on my tank as I do now!
I NEVER have to vaccuum the substrate with my DSB. I NEVER have any trace of nitrate in my tank. My specimans have NEVER looked so healthy and natural.
Yes, I am an advocate of DSB's. NOT for what they look like, but for what they DO. Not just for me, but for the creatures I love so much.
MY humble .02
Hermit