First the NSA, now the wife, what's up with people in HN? Are you all planning a coup with your mistresses?
If you're married and your SO can't just ask you for your PIN code - and you will, without even thinking about it, give it to them - you might want to take some time to think hard about your relationship.
I can't imagine why I'd need to ask my SO's PIN or she'd need to ask mine - I actually would, as you say, probably give it without even thinking about it; but if you have a healthy relationship then noone feels any need to check the other's phone/email/diary/whatever.
My wife and I have each others' unlock codes because sometimes we get info from each others' phones for one another. "Hey hon, I've got my hands full, can you see if Jane replied to my text yet?" or "Can I use your phone to look up X? Mine's about to die."
We don't feel any particular need to check up on each others' communications, though. It's just a practical matter for us.
Ok, there can be valid uses. But they haven't happened in my 9 years together with my SO. Probably depends on the habits - I don't call/SMS that much anyway; I know her ATM card code, as we needed that some times, but I've got no idea about her phone/other passwords.
How about a series of emails joking & griping about your wife's habits to your best friend? Completely harmless but not exactly the kind of thing that you'd want them reading.
Our digital selves trail back in time farther and farther these days; would you want your wife reading emails between you and an ex from before you met her?
Well if your wife has so little faith in you that she goes snooping in your email history, again, "you might want to take some time to think hard about your relationship."
If you're married and your SO can't just ask you for your PIN code - and you will, without even thinking about it, give it to them - you might want to take some time to think hard about your relationship.