Please allow backup of database in standby mode!
This comes up as a very common requirement in HA/DR planning. Logs are shipped over a (relatively) small pipe to a DR site, for days or weeks or months. As part of a (partial) failover plan, we need a full backup of a current version of the database to restore on another server on the DR side - but we cannot take the main DR (secondary) copy out of standby without breaking the log chain. Yes, we might have to pause the application of logs - or possibly not - in order to take the backup of a large-ish database. But the alternative is shipping a large backup all the way from production over that small pipe, which can be prohibitive.