Apple IDs were never meant to be used in that way. I realize lots of people do it, but that's not the way it was meant to be. The Apple ID is intended to be personal. Your stuff... Your contacts, your calendars, your apps, your music. The only reason to do it otherwise is to share music and/or apps between multiple people. Strictly speaking, this is a violation of the license agreement, though I don't know of anyone who's ever gotten dinged for it. You are allowed to put apps and music on multiple devices, but that's largely because of the assumption that you will not be using the same app on 2 different devices at the same time or listening to both your iPhone and your iPad at the same time.
If you have husband, wife, and child all using the same Apple ID, what happens when the child grows up and leaves home? They can't use your Apple ID forever, and they can't keep using the apps purchased on your ID if they start using their own...