Why the claim that copycat currencies are a threat to Bitcoin is a nonargument.
The value of a currency is the value of goods or currency that are demanded for the currency minus the transaction costs of that currency. These transaction costs tend to be so small that they may as well be ignored for established currencies.
Take for instance, paper money. Anyone can actually design and print paper money. Ignoring instances where people attempt to copy money, unique paper currencies not issued by a government are valueless for one reason: people are less likely to accept them, and it is more difficult to transmit them across vast geographic distances (no banks, no cheque clearinghouse). Both increase transaction costs, if one accepts the paper money, in order to purchase intermediate goods or pay for labor, one can either pay for it in paper money (which wouldn't be accepted), or exchange it for government money (which would be accepted).
You might note that there is a tautology there, value is linked to what you can exchange a currency for minus the transaction costs. If you can't exchange a currency for anything, then the transaction costs are high, and those high transaction costs will discourage people from accepting it because the currency has low value. The reverse will also be true. This is what gives the American dollar it's high value.
Now lets compare ACoin to BCoin. ACoin is accepted at as many places as American Express. BCoin is a new currency. ACoin can be exchanged for hard currency. You have to exchange BCoin for ACoin to get to government money. Which would you use?
