This is mostly a four year old thread now, but still a valid topic.
Four years ago when I last posted in this thread, my wife and I were sharing one ID with no problems.
Then my wife got an Apple Watch about two years ago. You can pair two watches with one ID but it means all the health data is merged. There isn't any way round this if you care about the activity elements at all.
So she now has her own ID.
There are two big downsides compared to how things were when had her ID....both solved with third party software:
1. Documents: When we shared ID, all our documents were accessible to both us. Now we have everything on Dropbox, which of course is accessible to both us with both of us signed in to the same Dropbox account.
2. Photos. With a shared ID we had one photos library syncing pics and edits from both our iPhones, and cameras, across all devices. Wonderful!. This has been completely solved by Lightroom CC which does the same thing, with both of us signed in to the same Adobe ID.
The last piece of the jigsaw was to add her Apple ID as a secondary account to my devices and vice versa which means Contacts, Calendars and Mail are all the same as they were when we shared one ID.
Family sharing doesn't cut it as it is not automatic and shared photos are low res, the family calendar is a single calendar, no scope for colour coding different events, and it doesn't handle Contacts.
Couples are all different and many won't want the same level of integration as we do, but many do, as this thread demonstrates.
So we are back to where we were but giving our money to Adobe and Dropbox instead of Apple.