It's not pedantry, it's just that RAID and instant data duplication or synchronization aren't meant to protect you from many of the situations in which you would need a backup. If a drive fails, you can restore the information from wherever you duplicated the data to. If, however, your data is corrupted somehow, the corruption is just duplicated over and you have no way to restore the data to a state before the corruption happened. If you accidentally delete files you didn't want to delete, the deletion is replicated over and, again, no way to restore them. RAID wasn't built to solve the problems a backup tries to solve.

If a program screws up and crashes while writing data to your drive, it can take out more than just the data it was dealing with. RAID will simply destroy data on both your drives at the same time, making any data recovery impossible.