I think I'll start to mark more often in level 50 dungeons.
Most DPS don't have a clue of what enemy to target first and they almost never care about what the other DPS is targeting.
It leads to situations where all 3 enemies of a pull (or more) will remain alive until they're all close to death.

This is quite an issue because killing an enemy faster reduce the incoming damage quite drastically, sometimes allowing the healer to start toing DPS instead of constantly healing.
Also, it's quite annoying to see the DPS starting to engage an useless enemy while another one, with a bunch of annoying AoE, will remain untouched (I just had the example infront of me yesterday in Snowcloak where the DPS would both hit two spriggans but never hit the Ice bombo, causing multiples AoE during the fight, thus more positioning, thus lower overall DPS).

So, yeah... since most DPS completly turns off their brain when it comes to targeting, I'll probably mark more often.
This is really sad since they should have these reflexes by now... but for whatever reason, they asume that they're "too good" to care about that and "it's fine anyway, things still die".

To be honest, I think I saw smarter DPS, prioritizing targets in a better way, during low level dungeons than in 50 ones.