As a healer I disagree with the OP.

Firstly, not all DPS-classes shine at AOE... and not all monsters are easier/faster to kill with AOE. Most times it is easier to focus down mob after mob.

Secondly, In most random groups its annoying, when every dps´er takes his/her own target and starts working in his own solo rythm.

Its a matter of time till one of the mobs wanders off towards either the zealous DPSer or towards the healer and the tank has to run after it... then he will lose another mob soon after and the real chaos begins.

Of course... if you have either a group that works well together or you have a group that is overgeared for the content... then it is a different thing.

But in random groups with content for your level of gear... you better get some order into the fights and that is easiest done by marking. (Be it from Tank or one of the DPS, dont care)