I agree with you. I didn't try to corroborate that Healer's Mercy is broken. I also don't think it is, having seen it in practice only just last night. It's very situational. Anyway, I was simply responding to it as an argument from someone else, trying to point out that BSoH is broken.
Just playing devils advocate.
Situation means useless 90% of the time and broken the rest. The unnerfed Turn Undead was situation but if you walked into a tomb it was broken. Same way if you are in a party with glass jaws BSOH can be broken.