With the dfa build you try to squeeze as much damage as you can inside that sod/dfa/dance window. The spec shines at fights with adds so you use sod to boost the aoe part as well because with a lot of adds (harjatan, mistress, etc) that part also does a lot of damage. And a sod/dfa macro is just easy to make. You could also macro dance with sod and spam that while dfa. Most of the time it is just lazieness.