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The single best benefit for AU scoring is the potential meteoric increase in AAOA, in my view.
This is the reason why so many...er...dubious credit repair firms offer those with 20 yr+ TLs with perfect history significant amounts of money to add an AU. Moreover, until Amex stopped this, an AU with Amex was even more valuable since the age became the AUs once he applied for his own card. There's been countless stories of people selling these TLs at $1,000 a piece.
For anyone building, rebuilding or just new to credit, a few 20+ year tradelines will translate instantly to a 700+ Fico, which will often result in an instant approval for initial apps, thereby bypassing the need for manual review or recon.
Shorter history AUs are much less meaningful.
*Edited* PS - This ability to "pass on" membership age used to be one of the best attiributes unique to Amex.