It’s a mix of shard generation and priority target importance.
But keeping things simple and just pumping numbers, in heavy AOE (8+) you want to be throwing out RoF’s constantly. Your shards go to RoF.
In heavy AOE the important part of Havoc is not the damage it is Soul Shard generation, because it doubles it, meaning more RoF’s.
Perfect situation would be having Havoc and an instant Soul Fire, that means 2 Soul Shards.
Another good spell to throw out during Havoc in AOE is Conflagate.
For like 4-6 targets yeah I’d still be using my CBs.