Like Shali commented, the best reason to have margin is because your funds are immediately available after you sell. In today's market and with the current economic conditions, I second his comment about having a margin account and NOT using the borrowing option. It is nice to be able to day trade and use unsettled funds if you want to.
Questions I wondered about when I first had a margin account.
1. You are NOT charged (interest) to use unsettled funds in a transaction.
2. You CANNOT buy every stock with unsettled funds. I have found you cannot by a lot of pink sheets with unsettled funds, but you can buy MOST of the stocks in the other markets.
3. There is a limit for how much money you have have in unsettled funds. I do not day trade that much, but I think it is 3 or 4 times your account value.
4. You can have a margin account as a retirement account. The catch is that it works more like a cash account because many of the options are disabled. Basically I am allowed to buy/sell with unsettled funds, but if I do my account is treated as cash for the next 90 days. I look at it like if I get in trouble I can get out, and then I have to play by the rules and be more careful for the next 90 days. I think it's kind-a dumb, but it works to help me out of a jam, so I am not going to say anything.
Hope that helps a little more.