In The Essential 55, Ron Clark says to err on the side of over-celebration. Cheering something only marginally worthy is less bad than not cheering something that deserves it.
Guild Wars 2 rewards everything and goes on to reward everyone for everything. Guild Wars 2 does not care if you level too quickly. It believes that it is better to reward your minimal participation. Arriving at the tail end of an event? Pitch in anyway, get some karma. You contribute one attack to someone’s fight? Full rewards!
Every other MMO I know works on the principle that xp is a scarce, rivalrous resource. Higher leveling speed could mean fewer months subscribed. Keeping people from receiving the unearned (and exploiting) is a greater concern. With xp as a limited resource per enemy, ensuring that you do not take more than your share is an issue of fairness. Most games limit unfairness through tagging and limit exploitation through additional penalties for outside help.
Tagging does prevent rogue solo DPS from exploiting tanks. For those who did not play in that era, there have been xp systems under which a tank could engage and take aggro, only to have someone with more damage swoop in and take most or all of the xp at no personal risk, and back in the day death penalties meant meaningful risk. Xp systems might reward whoever gets the first hit, the last hit, or does the most damage (or some more complex calculation).
Guild Wars 2 takes a different approach: everyone gets full xp. If you have done 90% of the work and see someone get equal rewards for tossing in one attack, that can grate. The leech has not taken anything from you, and even contributed a little, but still, unfair! That is an attitude we will need to get past. The current system encourages everyone to help everyone, even for very small values of “help.” Encouraging help only trivially useful is less bad than not encouraging help that could make a difference.
I have noted for years that, in MMOs, the polite reaction to seeing someone in a life-or-death battle with three trolls is to ignore him/her unless s/he explicitly asks for help. That is weird. I would much rather establish a norm of “everyone helps everyone,” whether you are nice or just greedy. It is pro-social design. We just need to develop a new concept with positive connotations in place of “leech,” with bonus points if it can incorporate “drive by.”