Yuri, you are forgetting one very important thing that can make marking targets (and focusing on one target at a time) better.

If there are 4+ trash mobs alive during the pull, the healer has to be continiously healing the tank.
If there are 3 trash mobs alive, the healer mostly has to heal, but can throw up an aero or bio.
If there are only 2 trash mobs alive, the healer only needs to throw out a heal every 2 or 3 global cooldowns.
If there is only 1 mob left, the healer does not need to heal at all anymore, and they can use all their DoTs and some ruins or throw out multiple stones on the last enemy standing.

Therefore, killing things in order, while, giving a slight loss of DPS from certain classes not using their DoTs, gives the healer more of a chance to attack things, which will easily make up for the DPS you missed out on.

Of course, this relies on your healer being good, not one of the lazy kind that just stands around and does nothing between heals.