Yes but that's like saying that Crit shouldn't be geared for because it can hit 100% and stop giving a benefit. Sha of Fear Heroic is a special fight because of the cooldown/mana reset mechanic essentially making a 20 stack going in be the same as if you had started a fight like that. It is outside the boundries set by a normal fight for stack generation and time to consume them. The fact that it CAN limit your maximum gain is irrelevant without the context of when that limit actually matters.