If you decide to park a database in read-only mode for a long period of time, go ahead and alter it into SIMPLE recovery mode too.
The message returned by SQL when you try to take a transaction log backup is:
"BACKUP LOG cannot modify the database because the database is read-only. The backup will continue, although subsequent backups will duplicate the work of this backup."
which is fairly self-explanatory to a human.
But to an automated system, such as a third-party enterprise backup platform, this message is not always interpreted correctly.
eVault, for example, does not detect the read-only condition or the message from SQL and instead returns this generic error message:
"A gap was detected in log files for database foo. You will need to re-run a Full backup."
Which is misleading and incorrect and pointed troubleshooting efforts of backup administrators in the wrong direction.