I think it's generally clearer to go with the numbers. If you're sticking with a single target yourself, then the DPS at least have the possibility of choosing their target based on what you're attacking, but if you're rotating your attacks between different targets (which can help you make sure you maintain aggro on all of them) then expecting the DPS to identify which target to focus on based on what you're attacking doesn't work so well.

Even if you do mark the mobs though, there are times when a DPS may deliberately cycle through them rather than following the numbers. If I'm playing a DoT heavy class like arcanist and the fight is easy enough not to put any strain on the healer, I'll cycle through all the mobs putting my DoTs in place. Ruin spam when all mobs have all DoTs is the only part that I'll focus on the first numbered mob, but while there's lots of mobs just maintaining the DoTs may take most of my time. Keeping the DoTs going on all of them is a bit more time efficient than Ruin spam in terms of how long it's going to take to drop the entire group of mobs.

(On a difficult fight, however, where it's more important to quickly drop the first mob or two in order to reduce incoming damage, I'll focus on mob 1 until it's down. I'll also focus on mob 1 if I've noticed from previous mob groups that the tank is having a hard time maintaining aggro against my DoTs-on-everything pattern. It depends on the group.)