I think for Arcanist, I definitely prefer laying DoTs across multiple enemies rather than DoT one enemy, then cast a happy 80 unaspected damage attack until it's time to refresh.

For Bard, I'll Windbite and Venomous Shot a second target, but mostly for Bloodletter proc against the first. I will rarely touch a third target with DoTs unless in one of the tank scenarios where I may overtake hate as the result of blasting a target. One exception is in Crystal Tower, between two Vassagos with a Napalm or Balloon add. There's a good chance Vassagos will keep DoTs while the napalm or balloon is the recipient of everything else.

But in most scenarios, am still a fan of mitigating damage/reducing stress on the healer where the downtime may shift toward healer MP if I take full advantage of DoTs and their durations.