This may be great in theory, but I predict it will backfire in practice. Can you imagine the gate agents trying to seat families together on a completely full flight when things went awry during booking? And instead of paging someone to the desk to ask if they would switch, they will do it unilaterally and you find out when your boarding pass beeps when you board? And of course the excuse will be that we have to change your seat so the family can sit together - you'll have no say-so whatsoever.. Can't wait to see those arguments at the gate.