I thought it was to keep the sport affordable to all. Not just for those who can afford multiple bikes. Also not all parents ride a bike so those that don't would then be at a disadvantage.
While we are on the subject when did it become acceptable for parents to go in the sections showing riders the line, kicking out rocks, trying to influence the observer, holding bikes in queues whilst little johnny looks at section, etc etc. And this is just at club and centre trials. I guess its worse at Nationals. From what i have seen some parents are getting too big for their boots and their kids are following in their footsteps!!
I thought only the rider could get the bike round the course and only the rider was allowed in the section unless to catch with the observers permission. The idea of parents on bikes as well. Not good!!!