Smoulder is good for PVP if you need to reach a certain AOE damage threshold. For example, if your Adamanti's first thousand strikes leave your opponents with around 10-20k hp, then having Smoulder allows you to kill some of them off the first round. Thus you don't need Herja and have to wait until the next round to finish them off, and you are not threatened by Flaviius. At higher levels of competition tho, that is unlikley since Smoulder damage seems to grow at a rate much less than hp and Adamanti's damage.