Hi Rosina,
I think the argument goes like this, for many people getting exp is simply a waste of time. They don't want to level, they just want to be high level so they can do the end game content. For these people leeching lets them skip the part of the game they hate and get right into what it is they want to do. Does this help the game as a whole? Probably not. But it does seem to enhance the enjoyment that they get out of the game; a very good thing for them.
I certainly understand your frustration with leeching as you are referring to it. Not the people opening chests, holding KIs etc because, as you say, these people are contributing something to the group. Though people in-game do refer to this as leeching I don't think you meant to argue semantics on this post especially since you specifically said these aren't the people you are talking about.
But as far as what multiple people being allowed to stay in group and afk for several hours on end is adding to the game, I'm not really sure I have an answer other than the end result of those people being glad they didn't have to level themselves. And let's all be realistic, people being able to stay in party with no contribution because the leaders aren't paying attention, they know the party leader, or for some other reason does happen.