There are some factors where it counts just as much, e.g. "Age of Oldest Account" (AOA). There's really no way to take a middle way on that. Either it's your oldest account, or not.
And AOA is in my opinion the only real reason to have an AU account -- if you are adding it to improve your score I mean. Obviously if the scoring effect is an after-effect of the convenience of using your mom's card, then that convenience issue is the real reason you are adding it.
With AOA you are getting at the classic example of how AUs are supposed to be used. Basically you have a person with no credit history of any kind -- not even one account -- typically a college kid. So historically it was a challenge to get a decent card without a credit history, which meant before you could get a good card you needed a card. To break that circle, your mom or dad could add you to a card they had had for 20 years with no lates. Hey presto! You suddenly had an AOA and an AAoA of 20! And it was very easy to get your first two or three "real" cards (or auto loans, etc.). Even after adding two more cards and a loan, you still had an AAoA of 5 and an AOA of 20.
Then around graduation, your parents could take you off their AU, and you still had your credit cards and your starter car loan. The next credit need after that you'd be able to handle on your own, because you'd already have a few accounts.
Where AU cards can often NOT count (dpending on the model) is in utilization. But trying to lower your CC utilization is a terrible reason to want to be added as an AU card.
Bear in mind, as AZguy mentions, FICO has implemented measures post-2008 to try to detect when a person does not have a genuine authentic right to be your AU (classic examples as parent/child, husband wife, etc. and the other side of the spectrum would be two strangers who live in different states). So once you get added, you would want to see whether your FICO scores change a lot. If they do, FICO considers you a genuine AU.
