Interesting, this is an issue we've struggled with from time to time.
We currently have our "own" money and are engaged to be married next spring. However, it is my suggestion that we share an account to pay bills from, and apart from that our money is separate.
Although we're totally committed to each other, I don't think it's wrong to have separate accounts. To me, it's not an issue of trust but one of independence. We are both, like-minded, independent people.
At first, she was a little put off by my stance (when we first moved in together for instance). Which is ironic, b/c her ex-husband bled her account dry. But anyhow, I like the idea of having my own money (and so does she). Honestly, I couldn't see it any other way.