I'd say AU helps my DH greatly. He is repaying his student loan, multiple lates, one paid CapOne CO (already taken off his TU - Yay!), and no credit card at all. I think his score was under 600. I added him on 5 of my good accounts around May (total $685 debt out of $14,000 limit), and his score has already improved, it's 640 given by DCU.
He was recently approved for a CapOne MC $500 (AF $39) and a CapOne $300 (No AF - no idea how or why they approved him this card). He was approved for Barclays Rewards MC $500 yesterday! Will be able to get his FICO TU from Barclays soon.
Side note, there is no option for AU on the HHregg store card. Anyone knows how???
It can stay on and for sure when removed as a AU, the affect of that account will be removed. i have seen both- the AU account still report as actIve and the AU must request personally for the reporting to be removed by the CRAs and I have seen them drop off immediately upon next reporting. Being a AU is a 2 way street... Account in good standing- good for the AU... Bad standing or maxed out.. Bad for the AU. 
Starting Score: 642