The history on that card crosses over to your own history. If it stays in good standing, you benefit. If something goes wrong, you share the hurt.
AU status was a perfectly valid category for married couples and others in long, established relationships. Both people considered themselves responsible for the card, whether as an employee pulling a paycheck that went to the card, or as the family financial manager who stayed on top of the bills and figured out how they would be paid.
Once it started being used by total strangers for profit, the whole process became corrupted. The changes in FICO 08 are a direct result of this. Once the changes finally take effect, many, many long-time AU's will be hurt, many of them stay-at-home spouses, because their AU CC's will not allow them to convert to joint status, and they will lose all their history.
No matter how marriages/ partnerships are set up these days, 20 and 30 years ago, an AU status was a perfectly valid way of handling family expenses, and these people are being screwed. Sorry for the OT!

As you might see, this is a topic close to my heart.
* Credit is a wonderful servant, but a terrible master. * Who's the boss --you or your credit?
FICO's: EQ 781 - TU 793 - EX 779 (from PSECU) - Done credit hunting; having fun with credit gardening. - EQ 590 on 5/14/2007