That's not entirely true. If your DRBD device is on top of LVM you can take a snapshot and mount this snapshot even on the secondary. We use such a setup for backups and it works just fine. You could even run PostgreSQL on this snapshot and do your reports from it.
Of course LVM snapshots have their own performance problems and just using PostgreSQL's native replication might be easier and faster, but still just using DRBD is a possibility which allows to cover not just the database.