The context makes it sound like they included the extra weight in fuel calculations, but it's not clear whether they included overall weight capacity. As the parent points out, this probably isn't worth it if they have to cut a row of paying passengers to do it.
An A320 burns about a half pound of fuel per second when taxiing. [1] So it seems plausible that the amount of weight you save on fuel could go a significant part of the way toward offsetting the weight penalty from carrying the extra motors.
I suppose if you're pushing the envelope precisely, and push back overweight, count on the taxi burn to get you down to max take-off weight, and plan to land with the legal minimum fuel reserve, then yeah, you'd still pay the full weight penalty. But I'm guessing that most airlines, as a policy, anticipate a range of possible taxi times in calculating their weight and fuel budgets, so significant weight savings should be realized.
Guessing that an A320 carries about 150 passengers, losing 3-4 of them would still make this worth it, and if they weren't flying 100% full in the first place the lost seats don't cost them anything. It's a neat idea, seems like a relatively safe mod and a way to catch some low hanging fruit.
smoyer, thanks for catching the weight. Seems like I have gotten a little too "efficient" with my reading.
Just carry a bit less cargo. I don't think passenger airplanes are typically limited by weight, but rather volume, when it comes to the number of passengers.
