This may be a slightly off topic, but I think an auto-number, auto-increment or user-defined sequence field type should be available at the geodatabase level, not just at the application level or UI level (like Attribute Assistant).
Here's a case: I programmatically insert some rows into a table that contains certified address certificates. The ArcMap user selects address points, enters some more information into a form and then prints the certificate. Before the print happens, I insert the certificate data into the certificate able using ArcObjects. AA seems to be hooked into the ArcMap interface, so even though I had set up AA auto-number, it didn't populate the field. So I worked around it by writing code to do just what AA does for autonumber: read the last number used in a setup table, add the next number to the target table and then increment the set up table by 1. That's a considerable amount of coding and testing that could have been eliminated by an auto-number field. Even the more 'lightweight' databases (Access, SQLite) have auto-number fields.
A third-party consultant once told us that ObjectID fields are not guaranteed to always be unique, especially in multi-editor environments, though duplicates are probably very, very rare. (That was ~8 years ago, maybe that has changed). But the problem with ObjectID's is that they'll likely get reset when a table or feature class is moved to a separate database (or copied within one database). That can make it hard to migrate to new versions of ArcGIS, create backup and testing databases and do regular GDB maintenance.
I know a number of people who find AA very handy and there's probably going to be some AA use cases for me in the future. But I think that auto-number should be available at the DB level. If I have misunderstood anything about AA or GDB's, then please correct me.
