Since the latest update (the one that broke the agro system) there have been so many discussions about it that I have yet to find an official reason to why the agro system was changed, if an official reason has even been stated. One unofficial reason I did come across was to help with people stealing mobs (DB hunting, farming, ect).
Whether this was the reason or not, it was a problem and still is. However, and easy fix for this is just to go back to how it was in retail. Whoever agros the mob first gets sole access to the items dropped (or credit for the kill - including 100% of the exp) for a short period no matter who does the most damage, or has the most agro, or actually kills it. The only exception ofcourse is if the original person to agro is in a party.
I doubt this was the reason for the agro update but is still a problem that’s been here since pre open beta. I believe this is an easy fix and is as fair a solution as possible.