Just spoke with my husband who is an air traffic controller at Logan. He said there are very rarely are their times that planes have passengers disembark through the roll up stairs. Although they often see passengers disembark from both exits on the smaller commuter planes that have the pull down stairs. He said that Logan has very few of those roll up staircases.
I am guessing maybe it depends on how busy the gates are. Maybe if the gates are all full and an incoming plane does not have a gate they will use the roll up stairs.?