Not only do you need to be able to access the password manager, but if someone cracks your password manager password, they

a) have a list of every online account you have and

b) now have access to every one of them.

Having a unique password and/or login to every site is one of those eggheaded ideas that sounds great on paper but has no real practical application in the real world.

I know I easily have over 100 online accounts in one form or another - there is no way that I could possibly have 100 passwords that I could remember. I would rather they have access to a small subset of my passwords than all of them!