If the users are never aware of other users and they only care about their own data, sure. Maybe you have a service where a user can sign up to do a specific ta

If the users are never aware of other users and they only care about their own data, sure. Maybe you have a service where a user can sign up to do a specific task that never requires interaction with another user. However, ff you want interaction across users (messaging, user-roles etc.) then you might want to have them in one database.