Lately, more and more games with group activities have been trying to hand out rewards based on the individual's contribution to group's success. The latest example is rewards for defeating rifts in RIFT. The problem with all these systems is that contribution is very hard to measure automatically. It's bounded by role, by level, and by strategy. Trying to say that Joe contributed more than Randolph, and thus deserves more reward, is a very hard problem.
It's even harder once players start trying to game the system. For example, RIFT apparently used the rate at which you used abilities to determine contribution. Which sounds pretty reasonable at first. If you constantly attacking, or constantly healing, you're probably contributing a lot. But players then started spamming buffs, or spamming abilities like "Track Minerals" to get to the top of the contribution meter. Obviously, Trion will fix this, but it illustrates how hard measuring contribution is.
I wonder if the better tactic would be to change the question. Instead of How much did the player contribute?, maybe the game should just ask: Did the player contribute? Instead of trying to quantify contribution, just make the question binary. And then just more or less equally reward everyone who did contribute.
I think the binary question is much easier to answer. For a game without defense, I would just use the damage done, healing or damage taken meters. If you are above a certain threshold of the top value on at least one meter, you probably did contribute to the group's success. It would be a low threshold, like say 20%. If you did more than 20% of the top damage dealer, you'd count as a contributing dps.
That's enough to give you a shot at the rewards, in my opinion. It may not be perfectly fair, but it's "fair enough", and all the other systems seem be able to be exploited more easily.
Now defense is much harder to measure. Let's say you're defending the mine in Arathi Basin, and no enemies attack you. You're still contributing to the group's success, but you don't show up on any of the meters. Maybe one solution would have the defending point give out a buff in a certain radius, and then have a meter count the time spent with that buff. A "time defending" meter. And then use the same threshold system as for the other meters.
So to conclude, quantifying contribution automatically is hard and vulnerable to exploitation. Reducing the question to a binary "Did the player contribute?" might be easier and fair enough to satisfy the player base.