Okay, consider this.
Yes, it's a fuel-saving measure, more recently implemented by American Airlines as part of their "let's not go bankrupt" scheme.
Here's what's to consider though.
The MD-88 is no longer a terribly efficient airplane. It was, in it's early day, more efficient than the DC-9s and 727s it replaced. But now, we have high bypass ratio 737s and A320s that are quite a bit more efficient than the Pratt JT8d-217's on the MD-80s. Even the -219s are not as efficient.
It not only reduces fuel, it also reduces the number of hours put on the engine. That reduces maintenance costs. Think, if they save ten minutes of running per taxi time, that's two taxi's per flight... if the plane makes 5 flights a day (which is a smaller number than I would expect) that's an entire hour of jet engine running you have cut out.
It really does save money. Now think- on a fleet of 400 MD-80s that American has, or even just the 150 (?) that Delta runs, that's quite a bit of fuel saved every day. That's maybe 400 hours per day (at one hour reduction per aircraft) saved!
Long live the MD-80.