It’s definitely convenient to be able to add and remove an AU and it’s definitely not possible with a joint owner, you’re right, you have to close the account typically. However, the joint owner is entitled to all the same benefits as the other joint owner, AFAIK. In my experience the benefits are the same, as are the responsibilities.
Another big difference is this, under FICO 8 an AU may not count for age, whereas a joint account ALWAYS will. Yes, I would not have wanted it as an AU. But it all depends upon your purpose.
It depends on the product.
With th the Altitude Reserve, the joint owner is NOT entitled to any of the same benefits as the primary card holder.
We had to pick who wanted the priority pass and there was literally no way to change that selection. Only one set of WiFi passes as well.
The only benefit we got was another credit card with charging privileges. The insurance coverages would have covered AU’s or family members anyway.
I will never again have an AU or a joint owner. Been there. Done that. Didn't like it.