From what Ive heard/read/understand is that a deep sand bed(5" to 7") will contain more benneficial anaerobic bacteria, which help "filter" water.
My last system was run on a DSB that was in my sump, kept in the dark(no lighting for the sump), this is also supposed to be good for lots of critters to live/reproduce/feed your sps.
Having your sand bed remote also goes well with having more flow
Not to mention if your sand bed "crashes" then you can remove it or clean it with less hassle.
Reefer addict, your onto something with having less rock in your system. If you look at this system this guy barely uses any rock. Not sure about all the details of that tank but... I had 50 # in a 75 gallon which is also way less than the amount recomended and my fish /corals/inverts did well.
http://www.zeovit.com/DTOTQ/edwardindex.html