They clearly had to be more then an ornament since they became standard on fighters. Spits came with the cupped mirrors that 51 drivers coveted. 38s had their own design etc.
I think it's a little much to say they were useless. If you read the history of the Spit, it started with none, pilots started to improvise, then a standard rectangle mirror on Spit vs and early IXs followed by the cupped mirror on the rest. Plenty of 51 drivers had two of them. You can find razorback jugs with 2-3 mirrors. While it may have been not much more then a security blanket, the pilots sure seemed to want them.