Why is flash cast kept as a secret?

I understand that the reason why Tatsuya can flash cast can be kept a secret since mental interference magic just screams yotsuba. However later on people start using magic without CAD, and I do not really understand the difference between magic without CAD and flash cast. I thought the entire point of flash cast was to use magic without CAD. However, if basic magic can be used without CAD, even if flash cast is better in any way, who would even be able to tell the difference? Why is this kept under covers?