I think what Gings says is the 2013 system.
each age grade will have a side selected to play the Nationals. I suspect this will come from FTC not the various SIF Tournaments.
More than 1 side per grade (obviously) increases participation at SIFT and also mitigates the fact that can we really pick the "best" 13 x 11 year old's for Nelson? Even if we could then I suspect this unbalances the tournament. The reason we don't pick A - B- C sides is partially to stop this happening.
For sure some parents and coaches much prefer the A- B- C type selections. I prefer the 'participation' style because by involving more children you are (hopefully) exposing them to a different type of football level.
For the grade for Reps I took this year there was 14-16 players clearly 'above' the others. Then another group of 10-14 all good , then another group of 18-20 all very even but only x amount of those guys can be picked due to the number of available spaces. So even with balancing sides players miss out who may rightfully feel hard by. If this is cut back to just 1 side then we have even more players "hard done by" and could well face a drop off in player numbers. Maybe.