I am only 46 PLD, but most of my exp comes from dungeons and guildhests (I do FATES while waiting in queue for stuff (not dungeons but guildhests :P)).

I understand that it's good to spread the DoTs and get them marked up. I've got a 50 BRD and having more chances to proc Bloodletter, all the better.
However, marking is good because it gives DPS a focus and better for your own survivability. 1 mob at 10% health is going to do the same amount of damage as it would at 90% health, so it's better to go through one at a time (if you're not doing SRs and have AoE out the wazoo). You can also think of it from a PvP perspective. Which is more effective? focusing down 1 player or man-to-man?