Percent util encompasses not only the affect on overall % util, which in this case is significant, but also individual acct % util, as well as % of cards reporting a balance.
The AU account, being sigificantly higher % util than your other accounts, is also hurting your individ cards util scoring. Plus, since it always carries a balance, it will always hurt your % cards with balance. Revolv % util is part of a 30% scoring category, rated much higher than age of accounts. While the util of one card does not make up the entire 30%, it is most likely at least on a par with the AAoA effect. AAoA is not the entire 15% of age of credit. YOur oldest TL is also a part of that mix.
You have no control over what the primary card holder may do in the future, so it is always a risk to be an AU.
I would also consider the intangibles. Creditor manual review of your CR. When they see an AU account, they know that it does not represent your own, personal evaluation of credit risk. Having no way to calculate a FICO score based only on your credit history, should they desire to do so, might put into question the enntire value to them of your FICO score.
The system may enable the credit history of others to be included in a consumer's scoring, but that does not necessarily mean that creditors will give it full credence in evaluating your credit risk. There have been prior attempts to remove all AU accounts from credit scoring based on such concerns.
I would drop it.
