In Europe both easyJet and Ryanair use 2 sets of steps as a preference to air-bridge where possible. The benefit is shorter ground times as it is quicker to fill the cabin from both ends than to fill from one end as people try and sit closer to the doors to get of quicker on arrival. Both of these carriers have high load factors and encourage hand baggage. Additional cost of equipment is minimal to an airline (although Handlers may have a different view as they provide it!).
Additionally easyJet removed the integrated air-stairs from the former B737 fleet before they moved to Airbus as a weight saving and a reduction in maintenance costs.
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