It doesn't protect her from people with a '$5 wrench' that want to get the password from her. This is assuming that 'they' figure that getting the password from her is easier/better than getting it from you.
Hired goon: "Excuse me, madame, but if you would be willing to divulge to me your husband's password, I will gladly give you this shiny new $5 wrench!"
(With apologies to "how to determine the height of a building using a barometer"...)
[By wife I mean legal spouse in general of course]