You can, but the experience sucks for families. If you want shared access to your family photos or music collection, you kinda need a shared account. But you probably don't want your family members to read all your email, so you need separate accounts. And what if you want to email a photo from the shared account? You end up having to switch accounts all the time, and the experience sucks.
The result is that people just use web apps like Gmail instead of native apps, because it's a lot easier to sign in and out of services.
The result is that people just use web apps like Gmail instead of native apps, because it's a lot easier to sign in and out of services.