Look at it this way. If one DPS attacks one target, it takes X seconds to kill him. If three DPS attacks that same target, it's dead three times as fast. On the other hand, if you have three targets and each of the three attack one each, they might die at the same time -- but they are also alive for the entire fight.
From a damage in-take stand-point, there's absolutely no benefit in having DPSers attack individual mobs. You are essentially asking them all to off-tank.
ಠ ,ಠ Dovie'andi se tovya sagain - it's time to roll the dice
As far as marking goes as a pally I only mark in raids. And that is usually only to let tanks know which mob to grab. Also I have it keybound to my numpad for fast marking. I remember back in TBC when CC was important, and marking was a must for CC and kill order. Now thanks to AOE tanking kill order is unneeded. I really kinda hope that Ulduar brings some of that back.