I don't remember that HBS comment at all! I'll have to go back and look.
I think the approach to Niki was in part a product of the times and the evolving public understanding of mental illness. I understand Agnes' intent and I understand fans loved Niki in the '80s, etc. But the fact is Niki and DID as explained in 1995 is exactly how they would come to be. It's not a joke, it's not funny. There is no other plausible explanation for (theoretical) DID - I knew that even as a child. It always goes back to sexual abuse, and Victor had been well-established pre '90s as a philandering man of secrets.
Do I think Niki could be sympathetic and somewhat dimensional - yes, I understand why fans liked that and I don't have a problem with that Niki as far as she went in the 1980s. I have a problem with her being presented as a semi-functional part of Viki that the rest of the cast of characters could all just roll with as a person in her own right. Do I think stories in the 2000s, etc. took Niki too far? Yes. They sometimes presented her sympathetically - a hallucination of Niki, the only 'mother' Tess knew, helping her give birth to her (miscarried) child in 2008 - but for the most part her stories were crass and ugly. Tess should never even have existed, and the Niki caper in 2002 may have been entertaining but should have been written for Tori. Though tbh I wanted Viki's DID completely retired after 1995. It should have ended there, period.
ETA: I did find the HBS quote. I would quote it but I only found out years later Jeff Giles was not happy we (mostly me!) quoted liberally from the book on here back in the day. I should have known better. I never wanted to hurt his sales, it's just a fantastic book.