There's no guarantee you will have a Soul Shard available. You can try plan ahead by always saving one but that will most of the times result in a DPS loss. Since it's completely random when the trinket procs you're forced to play reactive and ignore the possibility of a proc occuring until it happens. I don't have the trinket myself but I'm sure a situation people end up in many times is to having to choose between UA/Corr after refreshing Agony or just use Fel Flame to get all dots buffed (many times with a short duration because Fel Flame only gives 6s).