Why mark targets?

[MrDiezel]

Sleeping targets is pretty much the worst thing possible for damage, and is to be avoided. Party compositions are not made to always include a WHM or BLM with sleep, and the dungeons simply aren't tuned to require it.

A lot of techniques in the game are based off team composition. The purpose of sleep isnt to maximize damage, its to save your tushie. Especially if, like you said in your original post, none of your DPS have AoEs. DoTs can tick through sleep if applied beforehand.