You only have to watch the after race forum section of the TV show to see the sort of dumb-ass gooning that some fans get up to when they get into camera shot - not sure they would act any better if on the grid..
Someone suggested getting the track marshals to hold the grid boards - I guess they have never tried that task at a race meeting - there are a lot more important tasks they are doing in the run up to the start, for one trying to stop B/C/D list 'celebrities' getting themselves run over by teams pushing the cars onto the grid, or occasionally trying the stop rival TV cameramen/presenters getting into conflict over whose turn it is to talk to the drivers.
Are the grid boards necessary? - as someone said they help the spectators in the grandstand identify the start positions of their favourite driver. I would also suggest they may even aid the mechanics pushing the cars to orientate themselves as to where to stop, after all, with all the 'celebs' milling around, it can be hard to see grid markings or even see where your team personnel are standing whilst trying to avoid damaging the car by running over one of said 'celebs'.