It is not that simple In the case of a parity check detecting the drives do not agree with parity unRaid will not know which drive is at fault and the only action you can take is to update parity to match the drive.
If the write failed , but the corresponding parity write worked unRaid would disable the drive where the write failed and subsequently detect that the parity is out of step with the drive. You now have to decide if parity is right and rebuild the drive to match parity (normal action) or decide to rebuild parity to match the drive (which means parity now reflects any file system level corruption).
You can also get the case where a software or hardware error causes an incorrect sector to be written to both the drive and the parity drive without any apparent error indication at the hardware level. In such a case both parity and the drive are in agreement but the file system is corrupt.
Finally you have the case where you get more disks failing than you have parity drives. In such a case parity cannot help you, but often the majority of the data can still be recovered off a failing drive as long as it is still working at even a basic level.
All of these are low probability but in my view are still higher probability than the use case where encryption would matter to me.