Over the last "M&A Season", we have lost the following carriers at ATL:
- Northwest
- Continental
- Airtran
- US Airways
I've seen little benefit to having Southwest in Atlanta with a few exceptions, such as $99 1-way fares to DEN in the wintertime. But that's mainly just to compete with Frontier. With Frontier having changed their baggage fees, you can't really compare apples to apples to the other carriers into DEN.
Other markets with the removal of Airtran have become absurdly expensive. ATL-DFW RT regularly $400-700.
The European market has also really gone up. I've sometimes been able to find flights to Asia for cheaper then anywhere in Europe ex-ATL.