It's not just a 20% increase in damage. Let's look at the irregular and deep irregular essences. These increase tank damage output about ~90% iirc off the top of my head. That's an insane increase of dps, and makes tank throw out dps level numbers. More than essenceless dps. Similar essences exist for healers. People using these easily can cut a DR run from 45 minutes to 25 easy.