On multiple targets, I'd imagine it's way more profitable to AoE them with Fire II than to try to DoT any of them. Certainly at level 50, BLM is the dps job with far and away the most ridiculous AoE damage between Fire II and Flare.

Overall dps will be higher when AoEing multiple targets. Depending on party composition and gear, you may want to sleep stuff because it's safer. Single target focusing certainly kills individual monsters faster, but shouldn't be faster overall, esp. with BLM AoE potential.

I can't really speak to MNK, but I agree DoTs are only useful if the target will live long enough to experience the entire DoT. Only potential exception to this is SMN.