Its called rolling. No DKP earned, no DKP spent. I have a million DKP, you have Zero, I roll a 3 your roll a 4, you win.
Next, the bosses that have DKP earned and DKP spent are bosses we have down only a few times. The loot has not dropped on a fair distribution curve yet. I haven't gotten my 7.5 helm. Is there is a reason why I, who have been to every raid on KT for the last few months see the one item I'm looking for go to somebody who is there on the first time because they rolled a 4 and I rolled a 3.
Thirdly, loot on bosses that have not dropped at all or not on a fair distribution curve. There are a few people, fewer then you can count on one hand, that have not had the items that are best for their class drop at all. To tell them they now have to roll against people who show up sparatically or are on their first run is unfair, especially when they have been there since we started raiding?
--edit--
As for new area's like Uldar, you are not taking into account the loot rules. There may well be two druids in a uldar raid. You and a druid with 50 more dkp then you. The loot rules very specifically state you only get one item per run. So the first drop may go to the other druid, the next druid drop goes to you.
--second edit--
Lets take a look at how things will work for real newcomers, like say if Laraptor was to turn 80 this week and start raiding Naxx25 on this tuesday coming up. I am the only priest that shows up to raids over 95% of the time. I have all my gear from Naxx25 with the exception of my 7.5 helm. I'm not rolling against her for all the gear that she needs, because I already have it. So the fact I have 58 dkp is pointless. And then when we are on KT, and it drops the 7.5 helm token, I get it. Oh, Laraptor just got 5 pieces throughout the raid and I got one and I had more DKP then her. I really don't see how newcomers are hurt in the slightest. Actually, they benefit.