Regarding existing holders, if you have your own keys you are relatively ok (minus the obvious destruction of USD value), but the situation with coins in online exchanges and wallets will be "problematic" if say an exchange with 500k BTCs, say 'ok my clients, now you have 500k BTCCs because we adopted this fork' (and we are keeping 500k BTCs of the other fork for ourselves). It would be like stealing BTCs and exchanging them for Gavincoins.
People need to do a bank run in every exchange (maybe even online wallets too) well before we reach the point of the hard fork, to ensure that they have control of their BTCs.
That's fine. Online wallets and exchanges are not supposed to do fractional reserve banking. So there need not be a run, since all the deposited coins are available (you don't need to run faster than your fellow account holder, because the coins may run out). And if there is one who cheats -- al the better if that one is singled out as a cheater.
You're not going to stop spreading FUD, right?