High Priestess of Morrigan is not a boss, and many people don't care if they die to her, mainly because of idols, but boon does help reduce deaths from, her Chaos Storm, especially toons with low health.Never noticed a difference using or not using boon on any boss besides gele and apparently our tanks dont eitherI find any boss possible to solo heal excel for Proteus Prime and Gelebron. Prime needs 2 Druids and Gele 3. I like the details in this - also would suggest to add in EDL/DL boon on there for Proto, Gele, Unox, and Mordi. While it may be common sense, I feel a lot of druids, including myself forget to cast that skill a lot. It helps a sheet ton.
You've got to realize that players are over leveled so the mini bosses like Hrung, Mordris and Necro do less damage now, but back when 190 was the soft cap, boon was quite useful. Of course back then, the 3 minute cooldown was ridiculous.