Marking makes it a lot more efficient when you mark at least 2 targets. The main target and the next target to kill after. I notice it especially when I'm on my BLM. When I see the current target's HP is almost gone, I'm not going to start casting my next spell on it cause it will most likely die before I finish causing me to stop. So I instead switch to the next target and begin casting on that. With this practice, I can go through whole groups of trash mobs without ever interrupting my spells/rotation.

Not marking the first target is ok but if I have to wait to see which HP bar is going down, that's time I could have spent already casting my first spell. And if they don't mark the 2nd target, a lot of the times I just pick my own target. If I'm on the Thunder spell of my rotation, I wouldn't want to cast that on a mob that is about to die, that's a waste. So I cast it on the next in line mob.

Just my 2 cents but marking makes me a lot more efficient as a DPS as opposed to being lazy or what not.