I'm always amazed by threads like this where so many women talk about absolute trust in another human being. They are human after all and things can change.
Here's how I think about it.
If either member of the partnership decides to pull out today, where will it leave them? If both partners locked the accounts in their name, how would each survive?
For the high earner, their wage from that month could be automatically redirected to a different account. For the lower / non earner, how much will they have to use at the end of that month? This is especially true where there is currently nothing left over because if one earns but decided that month to pay no bills and to just walk out, the non earner would be really screwed.
We've read so many stories here on mumsnet where the low / non earner never saw it coming, had a wonderful trusting relationship until they learned their partner was cheating / became abusive / controlling etc and couldn't leave especially because they had no money of their own to get emergency accommodation anywhere.
When my exH disconnected the electricity on his way out to work one day during my maternity leave is the first time I thought it could ever happen to me. I cut my ML and went straight back to work. Thank goodness for the measely £2000 I had in savings. When he threatened to kill me one day, it was all I needed to get 3 nights in a hotel room with my DC and deposit and first month rent on a small house.
It doesn't have to be a secret running-away-fund. It should be discussed when all is well for each person to have a certain amount in their own name, should the unthinkable happen. It could even be a bank account compromise, IT problems, becoming incapacitated or whatever else.
No one ever expects it to happen to them.