If there's trust in the relationship, your partner won't want access. It works both ways. I can care less if my husband has access to mine and he feels the same because there is nothing to hide. ;)
Thought provoking question... Like you, I'm not sure that until something happens that a partner cannot be trusted, passwords etc should not ever be an issue. My husband can't use a computer, no face book, i have to retell him how to use the functions on his phone... that being said, when we first got together there was an issue of infidelity, and he copped to it very early on in the indiscretion (which is why we got by it). I think there are men that are untrustworthy and it takes time to find that out, i'm not sure that expecting every partner in a relationship to show passwords etc would uncover partners that are into subterfuge. Cheaters will find a way to cheat. Common sense is important. If a married partner finds it necessary to regularly go out with single friends to clubs drinking, etc. I wouldn't accept that. That would be the beginning of my trying to track his every move, and life is too short.. I think i might set my partner up with a friend that comes on to him when he's out and about, if this were the case. Someone to watch him while he's out with his single friends and see what comes of it.
I have my accounts and my husband has his and we don't know each others' passwords. It's not an issue for us, but I know for some couples this is a big issue. Everyone is entitled to some privacy and I don't think it's necessary that my husband shares all his passwords and accounts with me.
So, I have an archaic husband and he doesn't really have social media. No facebook or anything like that. I don't have passwords. He doesn't offer and I don't ask. He doesn't have mine. Or care. LOL We're like an old married couple (okay, we ARE an old married couple) but I know social media can cause a lot of issues and makes it easy for connection with others outside of the marriage. I think if my husband were super active with that, I'd maybe want to be a part of it.
That's just my thoughts.
I am not sure about passwords, but when I was in a relationship, I had access to my fiance's social media accounts. I would just go onto them from his phone. I knew his lock code. A lot of women think that they have to pressure their husbands or boyfriends for their passwords. That is not true. Things like that are given naturally. I never pressured any of ex's for their passwords because they always allowed me access to their stuff.