There is a lot of competition for USD so why would they go to you?
I know that on other exchanges (e.g. Mt. Gox) I can have a wallet in any of the currencies they trade. So if for some reason the XBT/EUR was trading higher than the XBT/USD (after factoring the EUR/USD rate), then it might be more profitable to sell bitcoins and get those EUR, then at a later time when the two markets equilibrate I then buy BTCs and move the funds over to USDs. So I might hold and trade with EUR even though I might never deposit or withdraw EUR. This adds liquidity to that market.
Does a Kraken account for someone in the U.S. allow wallets for both USD and EUR?
You can have a balance in any currency that's a reserve currency:
https://www.kraken.com/help/trading-guide#reserve-vs-non-reserve-currency USD and EUR are reserve currencies. You can't exchange EUR and USD directly, but you can deposit or withdraw in either and you can cash out digital currencies for either. But of course you will incur exchange fees (with our bank or yours) if you have to exchange fiat for deposit or withdrawal.
Also, you will be able to margin trade currency (i.e. currency pairs) you don't have a balance in with currency you do have a balance in. So, for example, if you only have USD in your account, you will be able to trade the XBT/EUR pair on margin using your USD balance. And margin trading does result in real exchange volume on Kraken, though no currency exchange happens in your account (you simply open or close trading positions in the currency pair). If you go to Account > Trade > Overview, you'll notice that your trade balances adjust to whatever quote currency (the second of a currency pair) you have selected in the upper right of your screen. Margin trading hasn't launched yet, though, so it will be a little while before this kind of thing is possible.
So there will be lots of opportunity for cross-currency arbitrage/spread trading. Ultimately this should add to liquidity, though short-term it may take volume a bit longer to get rolling with all the currency choice.