An important part of Shadow Priest's multidotting has always been the fishing for Procs. You'll put DoTs on as many targets as possible, even the ones you don't even want to kill, because multidotting always gave you something. Insanity, Orbs, Apparitions, instant Mind Spike/Mind Blast...
Multidotting was a key to keep Voidform as long as possible back in the days. So even if your DoTs deal pitiful damage, you'll want them on as many targets as possible (until the point where DoT refreshing hinder your DPS)