And you make the assumption that these heroes who can wipe out the entire zone single-handedly are dime a dozen, that every ally has two of them, and moreover, that such heroes exist. In my experience, most allies are just that, allies, a team effort of a dozen or more people working together to get the best EXP they can get. Some people have barely hit 75 and don't have much gear to speak of at all. Others were 75 before the level cap adjustments, and have some current gear here and there. Two guys wiping out the entire zone instantly when mobs pop? You exaggerate, sir.
Obviously, those with the best gear and focus on task contribute the most damage, yet everyone benefits equally in experience points. But that does not mean that those who contribute less contribute nothing, which seems to be what you contend. An unequal symbiosis? I'll give you that, but calling a level 90 in full Aurora a leech is hyperbolic, just as hyperbolic as these mythical champions that everyone else just leeches off of. What bothers me are people who go AFK indefinately and nothing is done about it. That's leeching.
Why bother leeching? Just buy accounts with real money, play with the jobs until you get bored, and then take up buying pre-built model airplanes as a hobby. After all, the point of model airplanes is having model airplanes, right?