I also believe that primary school is an important time to try to create some interest, however one of my major problems is that most primary school children have had free lessons. When the parents (or usually just mother in my experience) have to find the cash to pay for music service provision, different priorities conflict and the new year 7 child does not continue.
Maybe this is an instance where the first/middle/high school model is more successful. By keeping kids away from the "big boys" until Year 9 there may be a greater oppurtunity for retention into High School?
Would be interesting to see if this was true.