Sorry to disagree, but Nightline is a huge strength, especially in a time of war. It often beats Letterman, but has an older, more mature audience. Most important, though, it is great journalism. If you've never watched it, try it, especially on a night with a topic you know. You will not find better reporting and research, except maybe in big newspapers.
Letterman is great, and so is Leno. But Nightline is fine, too. We don't need to kick it out just to move Letterman.
As for money, Nightline actually makes a good sum of money for ABC. They are just risking it to try to make more with Letterman.
They seem to forget that Nightline's demographic (older, more educated) has advertisers too (mutual funds, cars, mortgages). They may not be flashy, but they are important, also; and in the long run these buying decisions will be advertised somewhere.
You can see discussions of that on the news links on the WDW main page, or on my other postings.