There may be a number of reasons for it.
1> One partner may not be too computer savvy and would never log into something alone.
2> One partners usage may be so minimal that it doesn't justify them having to manage yet ANOTHER username & pwd.
3> Not everyone really is into all this online identity stuff. It's superfluous to them. More than happy to throw out the rare comment on another ID.
Stuff like this - I'm sure theres many more reasons