My impression is that the narrative is casting the idea that letting go of all memories is bad, since you might find strength in some of them. But there's an overall thematic direction of the expansion and BFA as well that grievance against a party that wronged you is vengeance, and vengeance is bad. Hence why Uther has to be pushed by Devos to drop Arthas and later on, he draws a line between vengeance (bad) and justice (good) when it came to dealing with Arthas.