I think you bring up an important issue and consider the matter holistically. Overall, I agree that anything that erodes trust in a relationship must be viewed with suspicion. But I think you miss one important situation where it’s absolutely vital to have a secret financial account — in cases of domestic abuse.
Because there is so much violation going on, the abused partner MUST find a way to start building an escape route. Money is a big control lever for abusers, and financial abuse is a huge part of the overall pattern of abuse. In these cases, I would say that it’s absolutely vital to have that secrecy.