I honestly can't say exactly how much damage the elemental DoTs do. But I imagine it's comparable to Bio/Dia (Poison does more damage than Bio/Dia, but for a shorter duration). The fastest I've heard of anyone killing Old Six Arms -- a mid-level NM with a ton of defense and HP -- is 2 minutes, and that was done by a Conj using only elemental DoTs.

So I'd definitely suggest using them on NMs.