Just seems like the way they are going to execute the system will have no benefit. Everyone who participates to a certain level should have a chance at rewards, otherwise you are just making a system that encourages people to just hope people die for big monsters. With this execution id like to see what advantage they think multi claim will bring? What is their vision for this change, as they execute it?
Are you saying the system should allow anyone and everyone who participates to have a chance at the item/exp?

Because that's stupid. It'll just encourage people to mob around every enemy they come across. Parties would be non-existent. Idiots would just throw in a few punches at the last minute just so they can have a chance at the rare drops. Imagine spending a large amount of time and effort on that NM, only to have a bunch of people join in at the last moment and take all your stuff.

I think their intent is pretty clear: only the people who targeted the monster first will receive the rewards. Rewards as in all exp/loot.

Why is this bad?

Because certain players, particularly RMTs, will power level their characters by initiating mobs and then having their more powerful allies finish them off for them. There's so many ways this can be abused, and it throws off the game balance immensely.