I'll tell you, why I mark trash, from the view of new FFXIV and new PLD tank. I do it, because it makes the job easier for me. I'm learning in those low level early dungeons how to keep aggro, how to maintain proper rotation, how to make sure that all mobs are on me. It is helpful when I mark the targets, so the DPS can focus on one mob and then switch to another, because it gives me more time to react, if something goes bad, I mess up and loose the enmity. Sometimes it's just enough to mark the one I'm pulling, sometimes not... Also, if I'm in new dungeon for the first time, and I run with experienced players, I ask them to mark the correct order for me, as they know, which mob should be taked down first, as it does most trouble.
And on the other hand, as a new player, I've to say, that lots of people are really helpful. As soon as I state on the start of dunegon, that I'm new, they offer helpful advice, lots of tips and are doing stuff more slowly, to not scare me away.
