It's easier to hold any fraudulant owners accountable if you are an American account holder, for one.

Also some clients feel safer knowing that financial regulators of some kind are overlooking the accounts and transactions of the business they are working with.

I guess I should have been more specific. Other U.S. exchanges already have this, so what competitive advantage does bitcoins give an exchange?

If I'm reading you correctly: it gives the advantage that you can buy or sell bitcoins... which if you're someone wanting to do that is a significant advantage over exchanges that don't.

less risk of collapsing, maybe lower security risks (due to paying attention to regulations, but debatable).

It's probably easier to get faster payouts and the like as well, since banks will have less issue openly (snark intended) with a regulated exchange than money launderers.