I'm really not sure how to answer you here. The short of it is, that yes, obviously focusing on a single mob will result in more realized DPS, but our class is built to be a lot more versatile than to be relegated to a single target
I don't have any particular numbers on what Loading an exorbitant amount of DoTs on a mob would add (though my experience is that in place of Chaotic procs the number is negative), but Chain Nuking a mob would add about 15k DPS in the best of circumstances (assuming max aa and not factoring in the time that needs to be allocated to refreshing DoTs). Chain Swarm pets would add about 40k once established (with max aa, also factoring out DoT refreshing/chaotic) I can /sleep my way to upwards of 70-80k DPS (each) on 2-3 targets without burning/accounting for my pet. If the mob(s) are capable of completely negating our DoTs via regen, your DD rotation will hardly put a dent in them so why bother trying to make headway with a single mob when you can presumably hamper a second one's regeneration just as capably (Note that this is pretty much relevant only to events where we're engaged with multiple mobs at once, not kiting/offtanking them etc). Going swarm pet crazy will do a fair bit more but require you to essentially do away with half of your spell set, which will hamper that added dps considerably. So the choice essentially comes down to adding a tiny/perhaps respectable amount of dps to a single mob, or adding a lot to the overall encounter. I can't recall any mobs outside of crazy regen Anashti who can even come close to out regening our DoTs, this is especially true for add based events like Tita's Ghost where DoTing up every legalus is a heeeeellllllll of a lot more useful than following the MA.
It's hard to really come to any solid conclusion, as most of this debate is pretty circumstantial, but I've raided pretty much everything in EQ when it was either current or at least still relevant, and there have been very very few encounters with multiple mobs that I wouldn't endorse the tactic of Necros DoTing multiple targets