It's very useful for applications that don't guarantee API or data format compatibility between major versions. If you were running PostgreSQL 8, then under no

It's very useful for applications that don't guarantee API or data format compatibility between major versions. If you were running PostgreSQL 8, then under no circumstances would you want a silent upgrade to PostgreSQL 9, because 9 would refuse to read the data storage files from 8 and not start up: PostgreSQL required the administrator to handle migration between storage versions.