Patrick van Helden wrote:
Because then the data isn't consistent.
In what sense? As the PostgreSQL manual states, atomic snapshot + transaction logs together make a consistent database.
"However, a backup created in this way saves the database files in a state where the database server was not properly shut down; therefore, when you start the database server on the backed-up data, it will think the server had crashed and replay the WAL log. This is not a problem, just be aware of it (and be sure to include the WAL files in your backup)."