Yes of course they will know in advance, (as long as someone doesn't try to take their seat when they arrive, as someone else suggested on here), but as you say, the anarchic booking system and the inability to at least choose a row or block number, may put some people off from going.
A potential ticking time bomb in fact. It could also lead, in the long term, to some fans just not bothering to go to away games, simply because they have no idea where they will end up sitting once they get to the ground.